Two different cameras? or different settings on one camera?
Both are versions of H.264 so the frame size is only half the equation. The other half is the compression.
Look at the amount of video that can fit on a memory card, or how much an hour will use.
Cell phones at 640x480 video are highly compressed and can record an hour using 3-5 gigs or less.
1920 X 1080 HD camcorders use between 8 and 11 gigs/hour.
And good old standard def 720 X 480 DV (miniDv tape media) recorded a whopping 13 gigs/hour.
Sooooo without knowing the situation here, can't tell which is best. My guess is both are highly compressed and all things being equal, go for the bigger frame size. If you can, go for the one with the bigger data rate.
But if you are looking for quality video, stop. Do a little more homework, at least look at MiniDv (Canon ZR960 is about the last standing entry model).