It's up to you.
I found a desktop to be a huge pain in the ass. This was because if I had a break during classes I could pull out my laptop to do work, whereas that is impossible with a desktop. Sure, I could have gone to a computer lab, but computer labs just aren't the same as your own computer.
And profs definately let you use laptops! Most profs use them themselves!
You can check out the discussion that other students had last year about it here: http://www.macinsiders.com/showthrea...ghlight=laptop