the standard quad rail that comes on the MR and 416, is bulky/large diameter. I personally would never use that much rail space, so why pack it around. there is also a minimal (very small) weight savings. I went with the Geissele rail, as originally it and the Remington were the only games in town, and the Remington was priced like it was made from Unobtainium. the ability to attach a short piece of rail in the spots it is needed is handy.
Now since the Geissele and Remington rails, there has been additional companies offering forends for the 416 family. Midwest Industries offers a rail that has had good reviews on here, and there was talk of a carbon fiber or polymer (I don't recall which) rail being made by Lancer. I've not heard much about the Lancer offering in quite a while, so I don't know for certain if it ever came to market? Additionally, HK now offers a smooth round'ish Key Mod style rail, often referred to as HKey Mod, I have that on one of my MR's while I have the Geissele on my 416.
I can tell you the quality and build spec's of the Geissele are incredible, you'd think it was German/OEM it fits so tight and perfectly. I prefer the Geissele based on my experience with it and the original HK quad rail forend, mostly due to the reduced diameter of the forend. The one down side was I had to cut off the sling loops from my gas block. I was not really happy that had to happen, although it clearly had to be done to reduce the diameter as much as Geissele did. Ultimately, I wasn't too worried about it, as the damn sling loops never should have been on the gas block in the first place.
One improvement I'd have made on the Geissele, which others have done, I would have put a QD socket at the front end of the forend as well as the rear (where it already has one). I'm sure the forends were spec'd out requested a rear QD socket, but I prefer my sling to be attached at the front end. It's not a big deal, I fixed it with a Geissele add on QD socket, just would have been nice to have one built in at the front like it has at the rear.