I would go with 49 keys. I don't find myself using notes outside of that octave range at the same time anyway...with the exception of doing something with a classical piano/organ or splitting the keyboard for 2 synths. Sometimes I go a lot lower, but eventually you'll get more keyboards and you can just set one an octave lower
If you plan on doing anything outside your home/studio, a 49 is much easier to move around. If you're in a hotel or something and want to work on a piece, it sits nicely on most desks without needing to bust out a stand or put it on the bed. Additionally, midi controllers (at least mine) take abuse because you use them for your soft synth, might run them to your hardware, etc. Smaller ones are cheaper to replace.