informal = you enroll into a college and take whatever classes you see fit. You tailor your own schedule
formal = its a set program. you have to take the classes they force on you even if its stuff you don't need.
Which one is better? Neither one makes you more competitive, if you do well and get high grades, either option is good. Personally, I did the informal and like it better.
As far as taking CC upper level bios, IMO, this is a

Besides, I don't think many CCs offer upper level bios (the 300 and 400 level stuffs)