I like my 15 incher and plan to get the 17 inch with my next upgrade. The 13 inch is way too small for me--its harder to multi-task on such a small screen. However, I take my laptop outside my home about 3 times a year--so portability isn't a first concern for me. If it were, then I'd reconsider.
You'll love the 17". At first coming from a 15" I thought the 17" was big but after a couple of weeks of using the 17" it seems small again.
Now I cant tell the difference between the two. And its still extremely light when carrying it around.