I'm not familiar with those rails you linked, but if you get one of them I'd recommend one where the delta ring is removed and it only attaches to the barrel nut (either theirs or the stock one). This will free-float the barrel (which improves accuracy) and still provide a very rigid mount for the rail.
I do have experience with Troy rails; I like the Troy VTac modular rail:
I have the 13" one on a 16" barreled rifle and the length is perfect for me: it's long enough to allow a good forward hold but not too long. A 15" rail will be just short of being even with the back of your muzzle device; too long in my opinion (unless you want it to butt right up to a suppressor).
The VTac rail is great because it uses the original barrel nut but it still has a very sturdy mount. It does require removing the delta ring, but that's not too difficult. Keep in mind that it's skinny enough that some clamp-style gas blocks will still touch the inside of the rail. I originally used a Daniel Defense clamp-style gas block, but it touched, so I switched to a YHM clamp-style mount. It still touched when I flexed the rail so I ground it down a little until it didn't touch even when I pushed hard on the rail. You could avoid this by using a set-screw type gas block (they tend to be lower-profile), but I didn't want the hassle of dimpling the barrel for the set screws.