2 6x9 boxes vs 1 10'' box

[chevymotocross]

I have a pioneer head-unit and a sony x-plod 1000w amp. The 4 factory speakers have been upgraded to pioneer speakers. The headunit is in, the amp is working and all I had in the garage was some cheap walmart 6'' basebox or something... so yea I need to get rid of that. I was thinking either 2 6x9 speaker boxes behind the seat or 1 10'' sub. Which would be the best to go with? I listen to rock so I do actually want to hear vocals and not thump-thud-thump rap lol.