Whats the difference?

As you all probably know, Google will be selling the Galaxy S4 from the Play Store, a google experience version. Personally, its to expensive for me, but I have one question I can't get forget: What's the differnce between the "Play Store" S4 and an S4 from Att? (Or Tmobile?)

I understand that the Play Store varient will be an unlocked phone with an unlocked boot-loader, but once the At&t/Tmobile versions achieve that,(an unlocked boot loader) won't it be a very simple process to port the ROM over to the other version?

Warning: Complete programming/code writing nube speaking!