Meh, I was kinda in the same boat but I just bought the 13"
Reason being... if I am going to do tasks that involved a better processor, such as photo or video editing, then I would be foolish to get a 13" since a 15" would better suit my needs. With that being said, even the difference between the C2D and i3/5 is miniscal amount and I doubt you would be able to tell the differences unless, maybe, you were planning on doing major photo and video editing and I do mean major, like a video of a typical weekend.
I perfered the 13" over the others is because of the portability for *currently* due to school reason. Yea, yea, yea... some will say the 15" is portabilty but so the 17 for the same reasons as you might say the 15" is. Well, to me... 13" is barely pushing portability when you compare it to my old Sony SZ with a much lighter feel to it.
Obviously, you need to find what your needs are with the laptop. This laptop is 2.4 C2D and my old Sony was 2.2 C2D...and to be honest, the difference in speed is a HUGE difference. I optimized my laptop to run very efficient too, but this one feels almost twice as fast... like I am comparing a computer in the 1990s vs a computer in 2010 even the "difference is only 0.2"