I am more familiar with Maple, but have used Mathematica a bit. I am more used to Maple because I learned it first, and did my undergrad honors thesis in general relativity with it. The main things I use Maple for are simplifying gross expressions, finding the minima of gross functions, and solving integrals and ODEs. I used Mathematica last year for a knot theory project, because there's a very nice extension that let's you symbolically work with various knot properties. Overall, I would say that Mathematica is better in its ability to solve integrals and ODEs, while I do prefer the notation of Maple and I know the tricks of simplification a bit better. The general consensus, beyond my own personal experience, is that Mathematica is more complete.