It depends on whether price is your main driver. Clearly, if you're looking at a Smokin'-It, price is not as important as quality and awesome customer service...but if the hold-up between deciding between the 2D and 3D is price, then I think you've already made your decision. Otherwise, I'd suggest the 3D because it's better to have the extra room and not need it as opposed to those rare occasions where you need extra room and don't have it because you went with the 2D.
Also, if you go with the 3D, you can cook more than just one meal (if you're into that)... for example, while you're cooking up your one rack of ribs, you can toss in a rack of chicken thighs to smoke and shred for an awesome batch of chicken salad for sammiches later the next day or so...and on another rack, you could also be smoking some ABTs.
I'm not sure of the dimensional comparison between the 2D and 3D, but I used to have an MES 30 (and a 40), and I'd have to cut the rack of ribs in half in order to fit them into the MES smokers...and you can toss in a whole rack of ribs on one rack in the 3D without having to cut of some of the ribs. They're all going to taste the same, but smoking a whole rack instead of two separate half racks is more fulfilling to me. Just my 2-cents.