better to make a copy of the whole RTW folder, not just data. If you only copy data you have to rename it every time you want to play a different mod (it'll read the files in the folder that is still called "data"). And the files that are not in the data folder are few in filesize compared to the data folder, so it's not like you'll be wasting much more diskspace.

Mod-Switcher is a more elegant solution, but you have to be a little carefull with it cos if something goes wrong you still have to reinstall RTW (so keeping one extra backup folder could still be usefull)