I took my sweet time choosing a side, joining the Imperial Legion at level 47.
Things that led to my decision:
1. The Forsworn were in a peaceful state and were already negotiating with the Empire to be recognized as a separate kingdom/province when Ulfric suddenly came on a rampage.
The Empire is not a good little boy in that affair. They told Ulfric he and his people would be allowed to freely worship Talos if he dealt with the problem. So he did. The Empire then balked at the Thalmor taking exception to allowing Talos worship, and cracked down on Ulfric....basically they stabbed him in the back after doing dirty work for them. You can pretty easily extrapole this as being pretty much the primary reason Ulfric no longer has any truck with the Empire; he tried working with it and it screwed him in favour of pandering to the Thalmor to deny Talos.....and now he's the bad guy for 'cleaning up' with their blessing.
I did not find anything regarding the Empire setting Ulfric up to do that, all I found in game was he did it by his own decisions. I made my decisions based on what I found "in-game". Although I did not spend much time to do missions in Markarth.
2. And the dossier saying Ulfric is a Thalmor asset, albeit reluctant. In other words, he is a Thalmor spy. How difficult could it be to convince a power hungry guy? What if....
People continually bring this up and I have no clue how they can be so, frankly, incapable of basic reading comprehension. The document is hardly difficult to decipher, it's very explicit. The Thalmor captured Ulfric during the Great War and during usual processing, discovered he was the son of a Jarl of Skyrim. They thought this could be useful, so they told him that information he gave up under torture allowed them to take the Imperial City. This was a total lie. Everyone breaks under torture, it's a matter of time, so it's worthless to blame him for this, the real point is the Thalmor attempt to mess with him and make him an asset by making him thinking he was already a traitor (he wasn't, the City had already fallen, nothing he told them was of use). Such a person would likely believe they no longer have any option but to work with their previous captors because they're already a traitor.
This didn't work out. During the 'Markarth Incident' the Thalmor attempted to directly communicate with Ulfric....it doesn't seem to have gone well because they immediately considered him 'unco-operative' and his entire life now consists of turning Skyrim into a First Empire style Nordic military juggernaught that can turn Thalmor into quivering piles of hacked apart flesh.
The biggest point to take away from the dossier is that the only outcome that works for the Thalmor is the Civil War continuing. It completetly screws them over for either side to win. This is so the player can sit in their chair and feel all smug knowing that, no matter which side they chose, they stuck something large and blunt right up the Thalmors vulnerable bum.
I agree, but I still for the life of me can't seem to really like Ulfric and his self righteousness, talking about honor and such. The way he talks, that the Empire should have fought to the death for all the Nord blood spilled when fighting for them when he was actually one of the traitors during the war. Don't talk about honor and fighting to the death and never surrendering when he himself surrendered under fear of death or pain.
Thalmor: Hey prisoner, do you want to be king of a Skyrim without the Empire?
Ulfric: I'm listening
Thalmor: We don't really like you worshipping Talos, but you seem to really like it. We're winning the war, so if we win, will impose a ban on Talos worship...
Ulfric: You sons of ....
Thalmor: We'd like you to start a rebellion in Skyrim. Then we crush the empire from North and South. You rule the North and worship wheoever you like, then we get the South.
Ulfric: Deal.
That's...basically made up nonsense that ignores literally everything we know. And we know a hell of a lot, especially considering we're working with the Thalmors own unbiased opinion on the matter. Ulfric isn't a spy. That much is painfully explicit.
IT IS MADE UP. You kind of took it out of context.
3. The Khajit caravan story. When I was in Windhelm, some guy told me of the story of two caravans set upon by bandits. One was a Nord caravan, which the soldiers saved. The Khajit caravan was well, the Stormcloaks watched from the walls as the bandits attacked them.
The choice between the two sides is that the Empire has become an institutionally corrupt dump which even after 20 years had pitifully failed to restore itself*, but has a seemingly alright guy like Tullius in charge, against the Stormcloaks who are very clearly vibrant and eagerly willing to forge ahead, rebuild and generally bother to get out there and work for it, but are led by a guy who's frankly somewhat distasteful. So what do you want to choose? Help the Empire and hope it can reverse the decay, or allow a vibrant new Skyrim is rise up? People will, of course, feed you bulls*** nonsense that 'only the Empire can hold off the Thalmor', but that's an argument built on...uh....nothing really. The Empire itself is built on Cyrodiil having beaten the crap out of everyone else, there's no reason Skyrim can't become a mighty power....especially since people like pretending the Thalmor are unstoppable whilst failing to notice that their entire main army was completely obliterated at the Imperial City (Literally. It's hilariously rare for a battle to result in one side being killed to a man like that) whilst still having to maintain dominion over provinces being purged, and that the Summerset Isle is suspiciously the same size as pretty much every other province in the game. The Aldmeri Dominion are simply another Empire, but with Elves, and just as defeatable.
Ultimately, the choice is "which piece of s*** looks better to you". Because you're eating s*** either way. I'm blonde, so I prefer blonde s***. Every now and then I run through the game and eat Roman s*** instead. It all kinda tastes like s*** though.
Yes that's true. But when faced with the fact that at least the empire does not judge you for your race. Not even the Nords are completely behind Ulfric. True they hate the Thalmor, but the Imperials do to, its just that they're tied up in the moment in Ulric's rebellion. Actually, I forgot to mention that during the quest to kill the emperor for the DB (I finished DB before joining either side), what the emperor told me actually made me gain respect for him, and the empire. The choices he made were not the best choices indeed, and accepted death gladly, knowing full well that he made mistakes, and perhaps it was time for change.
And again, I was an elf, so no way I'm siding with the Nords in that play through.
*pro-tip, real life ancient countries and empires that aren't institutionally demolished easily brought themselves back to previous might in 20 years (often less), with the very inspiration of the Empire, the Roman Empire, doing this almost habitually considering that 15 years after ludicrously epic defeats by Hannibal, they were busily conquering Carthage....20 years is more than enough for a human population to have a couple of new generations. High Elves don't repopulate at the same prodigous rate humans do, if the Empire weren't completely broken down it should at this point be just as powerful, if not more, than it was at the start of the Great War whilst the Aldmeri Dominion is still attempting to replace it's armies in the middle of painfully holding down provinces it's actively purging. While it's most likely that Bethesda have NO DAMN CLUE when it comes to timescales, this isn't an excuse that can be used in-universe. The simple fact is that the Empire has utterly, pathetically failed to rebuild after 20 damn years. In such a case, considering the Skyrim civil war hasn't been on for more than a year or two and can't be used as an excuse, the Empire cannot be seen as anything but massively incompetent to the extent that it must be in horrific decline.
Thing is, this isn't real life. I'm just playing through and making my decisions based on what I see in the game.