Having used a few iMacs as my personal system over the years, I got an HP laptop and after 6 months, I couldn't stand it, so I just purchased a MacBook Pro.
My model is the 2.8 GHz 15" model. I chose it over the mid level MBP for the 500 GB HD and the 512 MD 9600 really, though the speed bump was also nice, though probably not that noticeable.
I like the 15" size as it's a good compromise between portability and yet is still plenty big for daily use. The MBP will be my sole machine as I am selling the iMac, and I needed the extra real estate.
Don't forget, as a college student you can take advantage of the free iPod Touch deal (which I did), plus you get a discount on the machine. I think it is $150 in the mid model, $200 for the higher model I bought. Pretty good deals right now.
Just to be clear, the main differences in the 15" models are:
Low End -
2.53 GHz Processor
Nvidia 9400m integrated video ONLY
250 GB hard drive
Mid Range -
2.66 GHz Processor
Nvidia 9400m integrated PLUS Nvidia 9600m GT w/ 256 MB vram
320 GB hard drive
Top Model -
2.8 GHz Processor
Nvidia 9400m integrated PLUS Nvidia 9600m GT w/ 512 MB vram
500 GB hard drive
The difference in processors, in all honesty, will most likely not be readily apparent in every day usage. The hard drive issue is entirely up to you, how much space will you need is something no one can really answer for you.
The biggest difference is between the low end version that has only the Nvidia 9400m integrated graphics card. This means that the graphics system shares RAM with the main system, and is not as good as a discrete card. They are by no means "bad" but for graphically intense tasks, such as gaming, you will see a LOT of difference between the two. The discrete option has it's own separate processor (separate from the CPU) as well as its own separate video memory.
The 2 higher end models come with both the integrated graphics and a discrete card. That means you can turn on the integrated chip when on battery power and save battery life, or you can turn on the discrete card for maximum power (like when gaming) but at the expense of less time running on battery power.
Battery is also the reason I'd say go with the MacBook Pro over the MacBook. The MacBook has a normal, removable battery. The MacBook Pros now feature a non removable battery. The biggest difference is that the non removable battery will power your Mac for much longer. As i sit typing this post on battery power, I still show over 7 hours of life remaining after already using it for over 30 minutes. Of course running a DVD or watching a fullscreen video or encoding MP3's would significantly reduce that time, it is still very impressive. In comparison, my HP would be lucky to surpass 2 hours doing the same tasks I am now.
So total time you can work while on battery power may be a big factor if you will be using your MacBook Pro on the go around campus. I still can't believe this battery, I still have the keyboard backlight on, my screen is at one tap under 50% (which is what I use even when plugged in, this baby is BRIGHT!), I'm on wifi and my bluetooth is still on, though I am running the 9400m over the 9600m GT right now, so that should help a little.
Sorry for the long winded post, but I hope that helps you. Love this notebook!