it's all about the real estate
go 17". not a question. i don't care if it's for just browsing or not. i rember in college when i bought my first Mac (7100 A/V, baby!), i got the 17" CRT with it. everyone in my class was SO envious & thought it was enormous. that thing looks like a big paperweight compared to my Cinema Display now.
point being, if you got the 15" right now, you'd be happy w/ it for a while simply b/c it's a new computer. after a few months when you have Safari, Word and iTunes all open, you'll be wondering where all your desktop space went. not only that, if you ever watch a movie on it, the 17" is widescreen.
suck it up & get the 17"...you'll be happier that you did...