There is a really huge amount of bonuses when use preprocessors, I recommend LESS. The best tool for automatic compiliation of JS, LESS, SASS, CS etc. with minification is freeware Koala (http://koala-app.com/
), just drag the folder and you'll get all the chosen files compiled on files' change.
If use Firefox, suggest Auto Reload plugin that will reload page on CSS compiliation complete.
I've chosen this machanism after testing a lot of other instruments, but only this makes things prompt and invisibly fast - save your LESS (I use Notepad++) and the changes are immediately appearing in Firefox.
LESS is more simple and useful in my opinion than SASS, it contains all the neccessary options constructed in a strong logical way. Some years ago it was not so good, but since 1.7 release I prefer this preproc only.
Suggest also learning LessHat library that will save you a lot of time.