I feel that screen space is more important than CPU or RAM when using Xcode, so if you're wondering which upgrades to get, make sure you pick the larger screen before anything else. But yes, I've previously used a 13" MacBook Air with 4GB RAM for Xcode development (and web development too) and it was fine. The only time I ever hit the limitations of the hardware, besides wanting more screen space, was when using Vagrant. A MBP will be perfectly fine.

That said, I'd always recommend an iMac over a MacBook if you're going to be in one place. An iMac plus the new MacBook or an iPad for travel is my ideal setup.

For the most part, I wouldn't worry about specs when buying Apple hardware. There will be some cases where you still have to think about it (gaming, say) but overall, the lineup is pretty clear. Get the MacBook or MacBook Air if you travel, get the MacBook Pro if you travel and need performance, get the iMac if you want a desktop and the Mac Pro if you want a performance desktop. And don't get the Mac Mini. Then, configure whichever you pick to be as powerful as your budget will allow. I can't remember how fast the CPU in my iMac is, nor what type of RAM it uses - and that's how it should be. Pick your computer based on what you'll use it for, not specs, then forget about it and get to work.