For portability, a macbook. laptops are getting more and more popular since they can be taken anywhere. if you're a casual gamer, a macbook should suit you just fine, the graphics chip is pretty good, but not for any "real" intense gaming.
macbooks are more expandable, easy to install a bigger HD, and ram, whereas the iMac just has a slot on the bottom for more ram... with a laptop, you could add a fairly inexpensive lcd monitor if you want more pixels.
and next time, this thread should go into the buying tips, advice, and discussion forum... just a note 😀