13" vs. 15" for MBP used as 2nd computer?

I have a Mac Pro with 23" LCD I use in my home office. I will now be working a couple days outside the home in another office, and am buying a MBP. I can't decide between the 13" 2.66 or the 15" 2.4. I lean toward the 15" for all of the performance advantages, but I'm concerned it's going to feel like a lot to lug around compared to the 13" (I've had 13s in the past, but never a 15). My ideal would be a 13" i5, but of course it doesn't exist - and sounds like it won't for some time.

For those of you that have had both, do you notice a significant difference in portability? What would you go for in this situation?