Hey, I'm taking my last class in college now. I had a desktop the first two years and a laptop the next two years of college. I would recommend going with a slim and light model around 6 lbs or less. I found that I preferred to study outside of my apartment/dorm, because for some reason there are just more distractions and I'm not as focused. My junior year I had a big 9 lb laptop and I just wouldn't take it with me, but now I have a 4 lb laptop which goes with me everywhere. I don't think I have to tell you which is more useful. I like to study at coffee shops, so bringing a laptop along is very convenient.
I think you should go with the lightest laptop in your price range, because all of them will be able to do WORK. Make no mistake though - if you get a laptop, you will only be using it for work (papers, excel spreadsheets, proprietary programs) because it will be pointless to play games on it. And you should buy one with that in mind (just go with integrated graphics, who cares what a game looks like on a 12" screen, not worth playing anyway)
Maybe that's a good thing. GPA(desktop) < GPA(laptop).
When you are buying a laptop, put your priorities in the right order. It is hard to know what you want when you haven't owned a laptop before. I learned the hard way that the most important thing is the weight, because what's the point of getting a laptop if you're not willing to take it with you? After that, for me, was battery life and small footprint (I use a fujitsu p5020d with modular battery - 4.3 lbs, 10 hr battery life, 10.6" screen)
My last piece of advice:
I think you should go with a PC for a couple reasons
1. the cheapest Mac laptop is about $1200 (12 inch iBook)
2. at some point in your college career, you will be using some type of software that is provided by your university. go with a PC for compatibility. some students who had Macs at my university were left in the dust.
I know that you are just going in for freshman year right now, so you probably won't take any classes in your major for a year or so. So this advice is with the assumption that you are not planning to get another computer in the next few years.
By the way, good luck with mechanical engineering. (I'm graduating in Chemical Engineering + Materials Science Engineering)