Just like almost any other brand, Corsair PSUs range from excellent to below average. Never trust a brand name, only look at the specific models. :wink:
550-650W will give you a lot of room for OC experiments. For comparison: I'm running a Xeon E3-1230 v3 with a GTX 780 OC on a 450W Seasonic G-450 PSU without any problems. Even with Prime and Furmark running at the same time, the system only draws ~380W out of the outlet. An OCed i5 will need a bit more power than the Xeon, but the GTX 970 needs a bit less than the GTX 780, so your total power consumption should be roughly the same (10-20 Watts give or take).
Should you ever wish to run SLI, a little more wattage may eventually be needed, although 650W is usually enough for two GTX 970s.
A bigger PSU won't give you real disadvantages, it will just be running at a much lower efficiency when the system is running in idle mode (browsing the Internet, doing Office work etc.), so this will result in a few extra $ per year on your power bill.
The Corsair CS is significantly better than the CX, but I just noticed it also uses a cheap sleeve bearing fan (which are not known for longevity) and rather mediocre quality capacitors. The other PSUs in my list are of better technical quality than the CS series, although the CS is by no means a bad unit.
The Seasonic G-Series, Antec Edge and XFX XTR are technically identical, as they all use the same and very good Seasonic platform.
Seasonic is one of the very few brands that actually build their own PSUs and sell them under their own name as well as being an OEM for other companies (others would be Super Flower, FSP/Fortron or HEC/Cougar). Almost all other brands like Corsair, Cooler Master, Rosewill, EVGA, Antec etc. don't build their PSUs themselves but use different OEMs for their units.
So, in my list we got three different units based on the Seasonic G-Series. The Cooler Master VS is also a very good unit built by Enhance, while the Seasonic X is already a high end platform. None of these units have any real disadvantages.
The Seasonic G and Cooler Master VS use a 2BB fan (dual ball bearing) and are semi-modular. The Antec Edge and the XFX XTR are fully modular and use an FDB fan (fluid dynamic bearing). Additionally the XFX is also semi-passive, which means the fan will only spin at loads above 20% or temperatures above 77°F, whichever comes first. The only thing worth mentioning is that the fan is a bit overpowered and tends to get very loud at higher loads.
Seasonic X is also fully modular and has a semi-passive mode.
Based on the current prices, the XFX would be my favorite choice if noise level doesn't matter too much. If it does, I'd go for the Antec Edge.
Edit:
Here's a review of the XFX XTR 650 which confirms what I mentioned above:
http://www.techpowerup.com/reviews/XFX/XTR650/
Edit2:
While gaming, your rig should consume around 300W, so the XFX would still be operating at a rather quiet level.