McCordRm, on 03 October 2012 - 00:14, said:
So I'm trying to decide between going with a new Desktop, or a gaming Laptop.
The big question, I think, comes down to video power.
What is the effective performance difference between the Desktop 680 GTX
and Laptop 680M?
I say about 20%, it's effectively a lower clocked 670.
It really does depend here... If you've got the money you can put together an awesome laptop but it will be heavier because of the GPU. But to me heavier isn't very bad on a backpack. So it's up to you.

I know people who have alienware kit and are very happy with it.
Keep in mind that these days, that GPU would last a lot longer in the current climate than it would have done 5 years ago. Tech on that and CPU front isn't moving ahead as swiftly as it was before (mainly because games don't utilize their power).