From what i figure the pluses of my Defy...
Rugged (I'm careful anyway)
Roms galore. We got Moto roms from 2.1 to 2.3 (leak) and very stable CM7 and MIUI.
Flash although i never use it anyway.
Pluses for OMC...
Modaco Community probably will grow to Blade proportions! I always preferred it here than XDA.
I am well aware i would lose flash support and MIUI. (CM7 would come eventually i'm sure!) But other than that i'm not sure what disadvantages there would be. I know they both have 800mhz processors however i'm not sure how they would compare to eachother really. I've seen the Defy OC stable to 1.3ghz. I doubt the OMC would go that high but that's not an issue really.
Just looking for opinions really for a bit of fun i know i haven't made my mind up yet. Both have an LCD screen too right? I know the Defy does.