13in Macbook Pro 2.3ghz or 2.7ghz?

IMPORTANT: Both the 2.3 and the 2.7 come as base models with 4 gigs of memory..Would it make sense to upgrade the memory to 8 gigs? I want a fast laptop... but is it worth it to upgrade to the 2.7 if I would just end up getting the 4 gig memory one? I'm thinking about getting the 2.3 with upgraded memory to 8 gigs.. idk how much of an effect the the memory has on the speed.