Here's my take on things:
If you pass the ATC or FC aptitudes you'll be pushed in that direction (cos not many peeps have the aptitude). Both good branches and exciting jobs, but promotion can be a bit hit and miss (small branches = less senior officer slots). FCs are slowly moving out of the bunkers and E3s are a great job.
Supply and Eng, good promotion and some interesting posting slots available in unlikely places! Also good cross-over prospects to civvy world.
Admin - hard graft, very office orientated and generally unappreciated by the rest of the mob. But can open up some spankingly well paid jobs outside.
Regiment - well you'd have to be mad wouldn't you?!!
Also, if you fill in all 6 boxes you'll be asked questions on all 6 of your chosen branches.
Regards, ADIS