If you can get both, that's the ideal. But if you can only afford one, read on....
For in class and around campus use, an iPad is more useable - smaller profile, lays flat on desk, not as visible to instructor, etc.. For more intensive use, like researching. typing and printing out papers - and there will be a lot of them - a laptop or desktop might be better.
I have students now who only use tablets, but they usually will have somekind of external keyboard, when they need to type a lot. Also you will need to have a wireless printer that is iPad compatible.
And then there are students who use tablets in class and where less typing is involved and then use an on-campus computer lab for more rigorous work.
Another consideration is software. A laptop is probably better, if you have to rely on using Microsoft Word, PowerPoint, etc. If you can get by on using Pages, Keynote, etc., an iPad would be fine.
And for times when you will have to give a classroom presentation using visual aides, remember to have compatible external cables to connect to the classroom AV equipment.