Can anybody tell me any differences between these two rail systems? Is there any notable differation between the two rails weight and size?

The only thing that looks really different to me from the pictures I've seen is the aparant aluminum heat shield in the KAC rails, but I haven't handled either of the rail systems so I can't be sure.

Can anybody throw me a bone here?

Here's some info on the MI. I was a little concerned about heat dissapation, but I'm here to tell you not to worry. There is no heat. The handguard is so well ventilated that no heat is noticeable at all.

www.ar15armory.com/forums/index.php?showtopic=2525

I just got my MI handguards today. They are actually pretty descent well worth the 129 dollars. I have a Knights FF on my other rifle and that well, is really good but that is FF. The MI hg is pretty solid but it does move a little but you have to want to move it. Installation was less than 2 minutes (took longer to open the box).

Rich  

The KAC M4 RAS also has a metal locking clamp that grabs ahold of the barrel nut and is held in place with a screw. This helps keep the rail stable and prevents it from rotating. The MI does not have any type of locking feature. But it does fit tightly.

The MI will take the KAC rail panel covers, and it is milled so the panels will lock in place properly. The MI will take the TD VFG. The MI are very solid, will not move with a VFG in place, and the rails are to spec. It is a great alternative to the KAC for much less money. And the KAC tends to be in short supply since the .mil is buying them all up at the moment.

Functionally it may be hard to tell them apart.

The MI 2-piece rails do have pins that fit in the teeth of the barrel nut to prevent rotation.  Not anything for you to tighten with a screw, but I think they work fairly well.  Mine don't move at all.