the 15" is only 1lb more. It's like putting a can of sprite in your backpack. It shouldn't be noticeable, but it kind of is, but not annoying. What makes it feel "bigger" is the larger physical size. You kind of feel like it is a large computer b/c it's physically larger, but in truth when you pick them up side by side the 15 is not that much heavier. After all, it's 1 lb.
The 15" does feel considerably faster to me, even when using chrome, etc. Programs open up faster, etc. I think even having dropbox or drive running in the background, having 4 cores makes the computer not slow down like it does on the dual core 13".
I'd say that if you never use a 15" and only start with the 13", fine. You won't know what you're missing. But go to the 15" and then back to the 13" and you'll be wishing for more performance. Apple will never make a 4 core 13" machine because it'd be impossible to make one that thin. A 13" 4 core machine could be made with the last design's thickness of 1" but Apple would never do that given their weird obsession with thinness as the dominant design principle and having all of their models have nearly the same thickness. 1" thick 4 core 13" machine would be too ugly for them, nevermind like most businesses would care if people wanted the machine. Ironically Apple's macbook lines make zero sense right now and proves that simplicity alone (sparse choices) don't make for coherent notebook choices. They should just make a pro have 4 cores, the middle range have 2 cores, etc. And come in both sizes.