Well i have my heart set on a Mac(i think) and i went to my school and used one of the schools Macbook laptop 13" and i think it's going to be too small for me. I was going to go with the Macbook Pro 13" but now am reconsidering. The only things i will be doing are: Office, Skype, Safari, bit of iLife stuff, itunes. Now i have a 23" dell monitor to hook up to and won't really be running around with it, but won't have it plug into the monitor all the time.
I've been playing with them side by side very regularly for a couple weeks (I've had a new unibody 13" checked out from the university library to do video editing, a friend who I hang out with all the time has a new 15" unibody). The MBP 15" is actually a relatively large and heavy computer. Its a little surprising. It seems pretty large even for a laptop with a 15.4" screen: Its definitely thinner than most competing 15" laptops, but as far as length, width, and weight, its on par to heavier than other 15"ers I've used. Part of that is that it has a much bigger battery than most laptops come with and gets much better battery life so you have to take that into account.
I'm not saying that the 15" is not portable, but its a substantially bigger computer.
As far as internals 13" vs 15" (assuming you can get an education discount), you're really talking about $1100 (for base 13") vs $1700 (for base 15"). As far as CPU, the 15" has the next generation architecture and is significantly (maybe 25% on average for the lowest models) faster. As far as GPU, with the 15" you get discrete graphics, again probably 30% performance improvement. RAM is the same. HDD imo is irrelevent because 250GB 5400rpm vs 320GB 5400rpm, if you're like me then neither option is near acceptable and you'll be replacing it yourself with maybe a 500GB 7200rpm or 640GB 5400rpm (either one ~$80 @ newegg).
One thing that only rarely gets brought up hear is that you will get significantly better battery life with the 13". The 13" has only nVidia integrated graphics, while the 15" has both Intel integrated and nVidia discrete graphics. Discrete graphics is much more powerful but uses much more energy. With the 15", there is no way to completely turn off the discrete graphics and the OS will turn it on even when you don't want it, and it really hurts the battery life. Apple quotes 10 hours for the 13" and "8 or 9 hours" for the 15". According to reviews, 8 hours would be about accurate as far as the computer actually achieves some percentage of 8 hours battery life that is consistent with the proportion of claimed battery life that most laptops get. So 15" should have ~20% less battery life.
My choice? If I had the money I would get the base 15" and upgrade the screen resolution, and like I said replace the hdd myself when I needed to. 1TB laptop hdds are coming. With the Core i5 and discrete graphics, the 15" is much more "future proof" than the 13" with the Core 2, which unfortunately is already obsolete technology. Walk into a Best Buy and check out the laptops, I doubt that they even have any that aren't on the new Core i3/5/7 architecture (to be fair, Core 2 Duo should be comparable performance to those Core i3s).