I'll start by identifying myself as a Computer Engineering major, so we share a lot of similar tasks for our majors.
I've owned a 13 inch, then a 15, and now a 13. The 13 is definitely better than the 15" for portability. You could carry the 15 with you every day, but it won't be fun, I can promise that. The 13" is much better for portability, and it is enough for web browsing/programming, etc. Just use spaces for when you need to multitask, or with the money you have saved, go for a large 1080p LCD when at your desk. The Core 2 Duo and 320M have plenty of power for the next 4 years.
Go for the extra portability and battery life, as the i5 is not a huge jump, especially since it is still a dual core. That's just my $.02, some people here may have valid reasons for disagreeing.
I admire your way in describing the usage of a computer. But remember that what the 15" has is actually a new generation of processors. Speed booster is one of the most advantages to have in a new computer. No matter what type of work a user might use, it's important to consider what technologies have reached since we are living a period that never stops developing.
As a computer programmer, I totally agree that 13" is fair enough to use as far of programming and other minor things, such as network analyzing and other similar things. Open source is what a computer scientist suppose to follow, and compete others by developing new stuff as individual or team work.
Of course, I would go for mobility and battery performance and of course I go for what my needs tend to be. I got my 2010 13", standard specs, and I'm thinking in returning back to Future Shop, and have a 15" instead. The reason for this action is actually after I realize that I really should spend my money either on something I will get the most advantages with extra several hundred bucks.