Everyone has given you great responses so far and I agree. Do what is going to make you happy. There's no telling how MS will affect you and it's just not worth trying to predict and not doing something that is going to make you happy.

I saw a doctor once. She had MS. She had one flare when she was 19 and never had another one since. She went to medical school and has been a doctor ever since. She's in her 40's now. There are many similar stories of MSers doing all sorts of things and succeeding. You can be one of them.

Whichever field you feel the most passionate about will reap you the most rewards. You can also combine the two - cancer research and neuroscience - and have the best of both worlds. Go for it Charlie!