I think L4D2 will sell more, but it depends on bundles
Uncharted has been out longer and has been bundled in multiple regions, it was also the only decent PS3 launch title to buy
L4D has a bigger console base to sell to and (apparently) a bigger FPS crowd who'll want it.
The only thing that may work against L4D2 was some of the users expecting more L4D DLC support to add more levels after launch and as such may be hesitant to hand over more cash.
Never argue with idiots
They bring you down to their level and then beat you with experience