The quadcore should be best for that -- quicker real-time response etc.. when there are more CPU cores to distribute things over.

It does depend quite a bit on the operating system, too though.

My MythTV box (Linux) is on a dual-core (quads weren't available back then). The two cores mean it can playback HiDef 1920x1080i while simultaneously running a commercial stripping job. Each ends up using about 100% of a core, but with two cores no problem.

Four cores would be even better there, as it could then run multiple commercial stripping jobs simultaneously (we frequently record multiple things at the same time, each of which requires a commercial stripping job to eventually run).

The recording processes themselves require practically no CPU time at all (hardware Mpeg encoders), so four of those can be active while watching something else and doing the commercial scanning.

Cheers