The way I understand it, Brave New world has most of what is in Gods & Kings, and if you skipped Gods and Kings you'd only be missing the maps and some other minor stuff...I think. --- http://raptr.com/badge/dc821beb3ce7e522d4d6721ba24c11e0/fs.png http://i.imgur.com/r5PgL.gif
I don't have Brave New World so I can't say 100% for sure. I'm just repeating what someone else said on these forums (I don't remember who though). I also bought Gods & Kings already so I can't even test it out for you by buying BNW by itself.
Gods & Kings adds religion back into the game and adds some civilizations, puts espionage back in the game, etc.... I don't see any mention on the store page of BNW having these features by default though...
I would get both if you want the complete experience... --- http://raptr.com/badge/arleas/uc.png http://www.speedtest.net/result/2868545294.png
If you get Brave New World, it contains all the mechanics, units, and buildings from Gods and Kings. The only reason to purchase Gods and Kings if you have Brave New World is if you want to play as the Civs contained in Gods and Kings, or want to play the scenario maps.
Your best bet, and to save money, is just to get the Gold Upgrade ($5.00 during a 75% sale, which is common) and BNW. Two purchases, and you get everything the game has to offer. --- "There must be some kind of way out of here, said the joker to the thief. There's too much confusion, I can't get no relief."