Stock Android is going to look much simpler than TW Android. That aside, TW isn't intrusive and it simply isn't just the launcher. It's packed with many features that Google's home made Android doesn't have. Which one is better? It's a matter of preference, to me, the S3 is the clear winner in that matter. On a Nexus phone all the menus etc. are going to be monochrome (Cyan) and it might be slightly "faster". Mind me, by faster I'm not saying it's going to be anything that blows your mind, and probably you won't even notice it in your daily usage.
Again, if you want a pure Android experience, you might try CyanogenMod 10 on your S3 (if you want to flash custom firmwares) and give it a try. CyanogenMod adds many feature to the stock Android, but it also has all the feature stock android would have, including the GUI, etc.