Hi,
I currently hold an A licence and am halfway through my B licence exams, the A licence is good and it does give you some responsibility and a few extra quid compared to being unlicenced. But the best thing would be to get the B licence, nearly every good contract job is for B liceenced with a Type.
And as was mentioned before you have to the same exams as A licence for B, only to a lower standard. The reason i had to do the A exams first was the company i started with, they would only bring us to A standard, the rest was off our own backs.
Best of luck with it!