No t sure since I've only played the first one. But, from what I hear though they may be really fun games they are not too heavy on the STORY. it's mainly there to give you a reason to do what you need to do. (Don't wan to ruin the game for you). Anyway if you are looking for a fun game buy the first one. You can buy it used at gamestop used for $18. Get the game of the year edeition, you don't need to buy it new since the DLC is already in the disc, if bought separately it would come out to $40 on xbox live marketplace. Also, Riptide is $40. Hope this helps some.
I've played both games and the second is a continuation of the first. They go well together. IMO the first game is better. Its bigger and longer and is more open world. The second is much more lead you by the nose and shorter overall. It's really an expansion of the first. I'd say start with the first one, but it's not required to get enjoyment out of the second one.