In the beginning, there was only the RIS and the RAS, neither of which were freefloating or stabile enough for optics.  The solution then was to use a special extended ARMS Swan Sleeve to connect the upper and the upper RIS/RAS rail.  It worked pretty well, but it did raise the height of the entire flattop, and some optics and such didn't fit right.

Then came the FF RAS, which is plenty secure enough for optics, and doesn't mess with the rail at all.  The only two downsides are the weight of the oversize steel barrel nut and that you can't get to the barrel, so no M203s.

Along came the ARMS SIR, which free-floated and was open at the bottom to enable the M203 to be attached, but like the SWAN Sleeve, covers the entire flattop.

Next came the RAS II, which is similar to the SIR, except it only raises the rail at a hump in the center, leaving most of the rail in front and in back of the hump at standard height.

Finally, the pinnacle is the URX.  It extends the top rail with almost no interruption and no hump, free-floats the barrel AND the M203 (nothing else does that), and due to it's one-piece aluminum construction, is lighter than a FF RAS.  The downsides here is that they aren't going to be available for a while, and they require the factory to install.

-Troy