Maple, because that is the only one of the two to which I have access.
I prefer mathematica. To me, it seems rather powerful and simple once you get used to the annoying commands.
I decided on male a while back because it had a more intuitive/ mathematical interface, and didn't make you use commands for 99% of its functions. however I have never used or seen a copy of mathematica so I could be wrong.
I have only used maple, and it takes some getting used to but is definitely a very powerful program, we used it quite a lot in my calc 3 class as far as graphing in 3 dimensions, and doing multiple Integration and/or flux-divergence work