Actually most games are shipped to retailers and rental buisnesses a week before release.mostly because many big box stores(bestbuy,gamestop,etc.)have to distribute copies to all their locations.and for buisnesses that sell through mail order(or rent),they will ship a few days early.So customers can receive their games by the release date.and sometimes the games will arrive to the customer earlier then release.Usually no more then 3 days early,though its not unheard of for ppl to get them a week in advance.But thats rare as a retailer could get in trouble with a publisher if they break street date by more then 3 days.
Just rent them both first.Then decide which one had the more fun MP experience in your opinion.and buy it.Though MW3 isn't gonna offer anything new gameplay wise.and GOW3 really is only bringing an upgraded horde mode and dedicated servers.Plus Infinity Ward will probaly not break formula with MW3.So you can bet it'll just be another cut and paste game.plus CoD is a released yearly franchise.So skipping this years release won't leave you out the loop.Where as GOW3,is the end of delta squad story trilogy.and if the GOW3 beta showed me anything epic is on track to release a quality game.
If your refering to codes like EA's online pass.Then there's usually an option on the game to buy.Also you will find them on Xbox live marketplace and/or PSN store.Remember most of these codes just give you acess to stuff you can play the game without.While others,usually yearly released sports games.Require them to acess online play.
The codes usually cost around ten dollars.Which for xbox 360 owners is 800 msp.But usually games that require a code will let you,play the online portion for two days.Or in Homefronts case you can play the online up to lv 5.Afterwards you have to buy the code.