My thoughts are to go with the flat top since you can do more things with it , other than that they are both good weapons .

Both have their good and bads...
DPMS has a bad rep with people for not backing their Warrenty's on parts or repairs and that changes depending on who you talk to on a daily basis. The only thing I have ever seen first hand that was negative about a Rock River while at a class was the staking failed on the gas key causing the weapon to go down. .

I have seen both side by side and as far as QC they are both clean, just check the staking done on the gas key and castle nut and either will be good to go. The RR might bring a small bit more if you ever have to sell it.

You will be happy with either..

Flat top

Flat top. A decent set of BUIS, good rail system(good is relevant depending on what you want to attach and why).

Amen HK. The only other difference between the two - provided they're factory assembled is that RR uses a two-stage trigger where DPMS uses standard mil-spec type hammer/trigger set-up.

You should check the staking on all carrier keys regardless of name brand because they're all mass produced and inevitibly you'll eventually come across one that was put together poorly.

Have a competent armorer check it out for you to check staking, trig/hammer springs are installed correctly, etc. and you'll be good to go.