It seems to me to be an example of the old trade off between power and portability. If you need/want power (and you mention your marked preference for a good gaming machine) then choose the iMac, whereas if you want portability, opt for the MB. My own suggestion would be to try a MBP; it has a lot more power and better performance than the MB and has an excellent screen, too, along with portability.
Cheers and good luck