I vote 2 10s vented. Generally speaking, rap is one of the few genres where vented gets more votes than sealed. My 2 10s vented foxbox gets pretty loud off 650 watts (in the neighborhood of 125-130 db). With the right subs and the right amp, you'll be happy. I think joefox recommends 2 alpine Type Rs with about 1000-1200 watts in a vented dual 10 box.
Sealed and vented have distinctly different sounds. Vented sounds/is louder than sealed all things equal. It's not AS tight sounding as a sealed box, but the increased output with rap is worth it. If you listened to a 50/50 mix of rap and rock, then it would be more of a debate, but if you want to show off with rap, the vented 10s box is the box for you. It's still got some pretty solid sound quality with the proper subs and amp matched up with a good midbass and frontstage. If you are going for loudness, you may want to look into sound deadening your truck's interior. You'll get some noticeable gains from a proper treatment of sound deadener, and you should also bolt the box down to that underseat storage area if you're going for output. At high volumes, mine used to get muddy sounding because the box would start "jumping" just enough to throw off the compression of the drivers. I screwed them down a few weeks ago, and it's another significant increase in output.