well if you get a $20 card you can get all of it. 20-9(3 mode pack)=11-3(all 3 preorder maps)=8-4(map packs a+b)=4. then you can save that $4 for when seige mode comes out. just remember you need gold for the extra modes.
All the versus modes are pretty good and there's a deal to get all three for roughly the price of two. But if you only get one of the versus modes, make it Predator. Survivors is really broken and Onslaught, while super fun, is more or less completely dead. Predator is really fun and pretty well-balanced.
I'd recommend all the maps as well. They're cheap and add a lot of variety.