Well, what it really boils down to is what exactly you mean by "DMR". If you're talking about an accurate recreation of a RS DMR, an M16 is more the way to go.
If you're worried about a plastic body (no real reason you should be, other than aesthetics), you could always get an aftermarket metal body (there's several on sale in the "For Sale" section, if you look around). The durability factor is not really an issue as long as you don't abuse the gun. Even a "full metal" (read: pot metal) body will warp or break if you treat it like it was made of steel or aluminum.
Now, if you're going for a gun that will shoot further than a stock gun at a high FPS (seems to be what the kids are calling DMRs), you've got a couple of things to consider: most games put on through this forum that have set rules and FPS limits that have a "DMR" limit (like the 6th Column monthly games) require that the gun shoot semi auto only. That means physically disabling the full auto function so that it can't be reversed in the field. Also note that high FPS set ups put a lot of stress on the gearbox. That means more modifications and parts that need to be purchased and installed. Also, you're going to want a hop modification to really get the most out of the gun. I'm not an expert on that, but you could talk with spazz or one of the other techs that do that kind of work.
All in all, think about the role you want to play on the field and go from there. If I were you, I would buy a used gun and go from there. Troll Craigslist. Some of the kids on there seem to have very little idea of what they have or you'll find a guy trying to off load his collection for cheap. There's M4s and M16s on there all the time at really good prices.
Good luck with the build and let us know how it turns out.
