if I am just going to put a acog on it and a flash light, the 6920 would be fine right?
I think I like the idea of being able to change up the rails on the 6920, than being stuck with what comes on the 6940...hmmmmm
anyone have any pics of some Colts?
An ACOG and a light would work just fine, but you should evaluate what you really plan to do with the rifle. If self-defense or other "emergency" situations is a potential use, then a quality red dot like an Aimpoint or one of the new EoTech XPS optics would be a good choice. Don't get me wrong, I love my ACOG, but its purpose is hitting man-sized targets hundreds of yards away.
You are correct in that the 6920 will give you more flexibility. Not only with the selection of rails, but with the barrels. From what I understand, the 6940 uses a proprietary barrel nut, and should you wear out a barrel or decide to swap it for some other reason, you're screwed.
Here's my LE6920 with Magpul CTR stock, Tango Down Battle Grip, Geiselle SSA trigger, Aimpoint Comp ML2 in a quick-detach LaRue mount, Troy Back Up Sight, Troy MRF-DI rail with Magpul XTM covers, Gear Sector Hand Stop, Surefire Scout with a TLS Head sitting in an offset Gear Sector mount, and an Advanced Armament Blackout flash hider. All slung up with a Blue Force Larry Vickers sling in quick-detach swivels. For a fighting carbine, this has everything I need and nothing I don't. With carbine length rails, you are limited with the real estate for placement of grips, lights and other accessories, but it is definitely workable.
Hope that helps!