Originally posted by KyJim:
The 15T version has a folding front sight and free floated front quadrail. The 15X has the A2 front sight (birdcage fixed front) and a quadrail which is not free floated.
Free floating the front rail is more expensive to install but allows for the mounting of optics on the front rail. If you simply want non-optics (flashlight, etc.) on the front rail, the non-free float is fine. The free floated rail is also supposed to be cooler when firing at near full auto rates. Not a concern for most of us.
I had wanted the 15T initially but they are hard to find. When I actually looked at what I needed, the 15X suited my purposes just as well. I put one in layaway. When I get it out, I'm going to put an Eotech and a sling on it. After shooting for awhile, I may add a flashlight and/or vertical front grip. None of these require the free floated front rail.