This is a simple questions with I'm sure a complex answer and tons of opinions.

If given the option between DPMS (flat top upper) and Rock River Arms (carry handle), everything else being equal, which one would you go with and why.

Rock River carry handle??? not detachable carry handle?
If not detachable and you want to add optics or pimp it out I would go with the DPMS then.

Have you considered barrel length? rifling twist rate?

If you just want to KISS, then the Rock River will do fine but more info should be taken into account of what your main use of it will be.

I am sure others will be along soon to help you out here.
Good luck.
LP

Take a look at Bushmaster firearms.

DPMS, only because I own one and STAG for the same reason. Both are reliable and top shooters.

I own 1 STAG ARMS and 1 Rock River both with removable detachable carry handle , if you happen to be a south paw / lefty shooter.. like myself STAG ARMS manufactures an upper made for lefties , ejection port and forward assist is on the left side as well.

I installed an ambidextrous safety on both of my AR15's, 1 with an EOTech sight the other with an ACOG.

Been Happy so far with both...

Left Hand
www.stagarms.com/index...Path=13_21
Left Hand Specs Video
www.youtube.com/watch?...ature=fvsr
Cool Vid with STAG ARMS AR15
www.youtube.com/watch?v=TgVhnWqAPLw

Also have a LMT upper
www.lewismachine.net/catalog.php

Build your own it's cheaper and will work the same .

I'm purely looking for an opinion based on is there a significant quality difference between manufacturer A and B and would the flat top on one be a deal break over the carry handle (yes detachable) on the other or vice versa.

With the exception of the above noted differences, the specs on the weapons are very similar. It's going to be a fun gun, third in line behind a Remington 870 and a 1911 for home protection, sort of weapon.

I can't build one cheaper since I am getting the weapon at no charge as a thank you. I get my choice between the two, just need to pick one and say thanks.

I have a Rockriver Tactical Entry & have never had a problem with it for what it is worth. I am very happy with it.