Someone else will be able to answer your main question....but I just wanted to make sure that you know it is fairly simple to install your own ram in either of those computers (and significantly cheaper).
I was under the impression you could only buy BTO machines from the online store, but I'm not sure. The Geniuses will install new RAM on a machine you already own, though.
You can get a RAM upgrade at an Apple store. As Drile says, it is easy and cheaper to do it yourself. Other than RAM, the stores generally only carry the stock configurations.