^ This, but I would get the 15". Not because the i5 processor will be loads faster than the C2D, but because of resale value. the 15" seem to be holding their value better than the 13 for some reason. SSD prices continue to fall, just order the stock 15, buy an SSD and 2.5 external case. Install the SSD and use the HD as your portable backup
Even if it was impossible to swap to an SSD later, I would still get the 15 inch. More screen space, etc. Just a more valuable computer. (Though I am still very happy with my 13 inch. )