Just personal opinion, but I think that the DD is a better rifle than anything that RRA makes, but of course, the .308 is a whole different animal.
My limited personal experience with the RRA .308 is that it shoots as good as any other in its class. I'd say that the big decision to be made among AR .308s is which magazine do you want to commit to.
Do you have another AR-15? I wouldn't sell my only AR-15 to get into an AR-10 type rifle.