13" and 15" are different animals. Aside from the performance difference which isn't all that much unless you require your laptop to be capable of high performance video editing or something, the biggest factor in your decision should be whether or not you want something heavier and larger or smaller and more compact. I had the 15" for a little and found it too big for me to lug around. The higher screen res, better speakers, better CPU and graphics card was all good but not necessary for me. The 13" did everything I needed, had more battery life and was more mobile. Plus if you get last year's model used it may be out of warranty.
Id go 13 but that's just me, your needs may be different.