Which MacBook Pro is sufficient for Xcode?

If I were you, I would look at non-Retina 15" models. I've no problem running Xcode or any of the Adobe CC apps.

Clinton

XCode will run on any Mac capable of running the OS X flavor you want to develop for. Latest, 4.6.3, can develop for 10.8 and below, as well as iOS 6.x. Yes, the same IDE is used to develop both for Mac and mobile.

When Mavericks and iOS 7 come out, they will require XCode 5.

When the time comes for results and you're doing the final build on the long expected product, a faster computer with more RAM will naturally compile, assemble, link and produce the finished app quicker, especially if it is a large and complex product. Else, a little patience goes a long way.

While on the road to that magical moment when the efforts bear fruit, a larger screen affords the ability to visualize more lines of code and have more windows open on your forthcoming creation. Not suggesting an insufferably expensive and unserviceable Retina, but the Widescreen/Anti-glare option on the 15" regulars essentially gives you half-a-Retina with a non-reflective screen that's way easier on the eyes. For a whole lot less.