Yep, what DrBone said about the horizontal strip on the 2.3. Two versions ago (Skinny v Skinny 2) is super easy to distinguish because of the zippered lower disc compartment.
My son has been using his Skinny 2.3 for about four months now (probably used it for 30-40 rounds) and it's holding up great and he seems to love it. Small, light bag, can hold from 12 to 24 pretty easily, go with fewer and you've got a nice amount of storage space, cheaper than the other good alternatives, stands upright like a champ. It's a really good flexible design imo.
Go back a few pages and see pics of the wooden box I used to break up the storage area . . . that makes it fantastic. They should (next generation) add some velcro and disc compartment dividers imo (like what the compartments on the Rebel or Fade Tourney uses).
I'm still using my Shift and it's good too but if I had to choose between them I'd probably go with his Skinny 2.3. I'm glad he has it, I often prop my Shift up against his Skinny. lol