I don't know if there's any features the CSE can't do you'll need the old CS for, but for everything else the CSE is more stable, more comfortable, and has more features than the original.
Luckily, it is also just like every other OBSE plugin/-dependent mod. If you start the game without OBSE, everything is back to Vanilla. If you start the CS without OBSE (not using the obse_loader.exe shortcut or batch file to run the CSE), it will start the Vanilla CS. There's no either-or, they can both be used at any time and switched at will.
If you plan on creating patches, or using other mods as masters for your's, you'll love the CSE's ability to do that right off the bat, unlike the CS where you'll have to turn the master ESPs into ESMs first for it to even stick, then back to ESPs before running the game, and rinse, repeat every time you edit your mod. And if you're going to use OBSE function calls inside your scripts, well, you'll need to run the CS from OBSE anyways, and the CSE is more comfortable when it comes to OBSE scripting, too.
I for one am using the CSE exclusively currently, and I haven't yet run into any obstacles so far.