The 6940 (bottom rifle) has a monolithic upper (the rail extends as one piece all the way from the receiver to the front sight); the barrel is free floated. It also has factory installed flip up front/rear sights. Meh...
Unless you intend to mount some serious target acquisition hardware to a personal weapon, I find the whole concept of masses of extended rails to be worthless added weight.
A 6920 (with removable carry handle) will take any Aimpoint or other scope you are likely to use and seat it right where it needs to go...on top of the upper receiver... and still have room for a rear Back-Up Iron Sight (BUIS). Of course the gun will shoot pretty damn nicely with the factory iron sights it comes equipped with. The only other thing you might need is a weapon light mounted up front, which can be mounted in any number of ways to conventional hand guards or aftermarket hand guards (like Magpul's inexpensive MOE furniture).
Multiple rails were designed for military users to be able to mount multiple lights, scopes, lasers (both visible and IR), grenade launchers, and the inevitably required vertical fore grip for tape switches and pressure pads from competing devices. If you don't own all this kit...and the helmet mounted night vision monocular to go with it...why bother?
My issued Colt M4A1's rails mounted with a moderate load of stuff while in Iraq...
The 6940's monolithic upper rail provides for more rail-estate for mounting combat optics, swing-out magnifiers, backup rear sights, and night vision/thermal weapon sights along the same plane. This represents many thousands of dollars worth of optics (that you will likely never own) and turns your light and handy AR into a weapon with the weight of a WWII BAR.
You don't need it. Go with the 6920 (or BCM equivalent). Add a Surefire or Streamlight TLR-1 flashlight with an inexpensive mount that you can operate with a thumb or pressure switch. At that point...you are good to go.
It's a carbine...not a Swiss Army Knife.
I have been priced one at a gun store with the markings for 1595.00 cash/1695.00 credit for a marked le6940.
With regard to the LEO Restricted markings on previous 6920s...think of it as last year's model. All they have changed on the gun is the roll mark. Certain unscrupulous dealers are now trying to convince naive buyers that this somehow makes them "special" or "collectible". They aren't. Colt will continue to push LE marked lower receivers out the door until they exhaust their stocks, but all newer production simply has a different roll mark.
Shop Around. Colt is now pumping out several package variants to dealers with most of what you might want in terms of features.
Like this one...railed M-4 profile:
http://www.armslist.com/posts/29498...t-le6920mpr-ar-15-limited-magpul-talo-edition
Or this one...railed lightweight (M16A1) barrel profile:
http://www.armslist.com/posts/30818...le--colt-lt6720r-magpul-limited-edition-ar-15