Mathematica and Matlab differ on their aim, so youcan not choose one or another, if you want to perform symbolic analysis mathematica is a powerfull tool, but if you need to work with data matlab will solve everything you need.
You should learn both .Like tha last person said .One is powerfull in symbolic math .The other in numeric computations .If you do theoretical work .matematica is a must .if you do engineering ,welcome to matlab.