Something to consider, is that although the 15" is a couple inches bigger, it will weigh more. So if this would be an issue, you may want to consider the 13". However, if a bigger screen is more important, then go for the 15".
For what you state you'll be using it for, I doubt you'd see much benefit from the quad cores.
There's quite a price different between the two versions however, the extra cash you'd save in buying the 13" could go towards extra RAM, external HDD for back up or software.
Having said that, if money is no object and the weight isn't an issue - Get the best one your cash will allow.