I have the Optimus, my wife has the Defy. Defy is faster at rendering web pages and has better battery life. Optimus seems to have a better cellular radio (stronger cell signal), and is rooted so that you have better potential application management. I'm staying with the Optimus for a couple reasons. Number one is that it just works well for me, number two is that I have coverage at my vacation spot where I wouldn't with the single band Defy.
This might be helpful: Motorola DEFY XT vs LG Optimus S - Phone specs comparison
Better screen, faster CPU, newer OS. The specs show the camera is better, but the camera seems like a weak point on the Defy.
I think the biggest reason to switch is that the Defy is a platform that should be receiving updates from RW. They've implied pretty heavily that the Optimus is a dead end and it's unclear what improvements will get pushed out to that platform.