Definitely laptop!
Computers are a commodity these days, so don't go crazy. I spend lots of money getting the newest and best during my earlier years in college. During my senior year, I did tons of intensive engineering work on a Thinkpad I bought for $350. It handled everything like a champ.
I would completely echo the comment made earlier: buy an inexpensive, used Thinkpad! It will last forever, you won't feel bad if you manage to kill or lose it, and the performance is great. I used a T61 for most of this year, and just "upgraded" to a T60p with the IPS UXGA screen.
There are lots of kids in college who spend $2k on a MBP, only to use it for typing English papers and surfing Facebook. On the other hand, I was using my $350 Thinkpad for heavy CAD duty (T61 had a Quadro, T60p has a FireGL), signal and image processing in Matlab, typesetting big reports for senior design in LaTeX, and a lot of other things. Make the most of what you have, and if you find that you somehow really need more power, upgrade without breaking the bank.