AutoDetect, or as some call it , autodefect, more often than not defaults to MTP. You need MTP for subscription services like Rhapsody, and people usually have more success transferring playlists with MTP as well. MSC, on the other hand, makes your player appear to your computer like a thumb drive. Another benefit of it is that it allows your Sansa to work with just about any OS...Windows, Mac, I think people even make them work with Linux(i know nothing about Linux). Also, with MSC, since it's like a thumb drive , you can use My Computer/Windows Explorer to drag and drop your music onto your player, if you don't want to have to use WMP,WinAmp,MediaMonkey, etc. every time you add/subtract music if you don't want to. Biggest problem with AutoDetect....before I forget...anything you put on in MTP you can't see in MSC...and vice versa. So you're better off sticking with one or the other.

My players:4gb Fuze,8gb microSDHC,2gb Clip
My 'phones:Sennheiser HD201,Koss KSC-75(x2),Sony MDR-V150 (major EQ adj. required)
My amp: FiiO E3