Think, would 320GB be enough for you? Do you already have a bunch of files that you know you'll transfer to your new Mac the moment you get it? Will these files fill that (320GB) storage up significantly? Do you expect to use up that space (320GB) quickly, in the course of your use of the laptop? If you answered yes to above questions then go for 500GB for an extra $50. Otherwise, I really don't see the benefit of buying storage that'll never be put to use.
I, personally, did CTO my 2011 i5 MBP with 500GB HDD for an extra $50 but I did so because I already had a 120GB iTunes library, 20GB photo library, and was expecting to run 2 Windows 7 VMs (each being ~30GB in size) and a few games (AOEIII, WoW, Sims3, etc.. totaling another 40GB).