new-arguy pretty much summed it up. A couple other benefits, on the SIR-50 you can co-witness an Eotech and place the Eo out over the barrel. You have flexibilty as to where you want your rails to be and not to be. Also, the system comes standard with a side sling swivel, an extra cost with some of the other systems I've seen.
I like mine quite a bit. When the #50 was announced, I had MSTN get me one as soon as they had them in stock. I've used a RAS equipped rifles, various Fobus setups and some free float tube setups and the SIR system fit the bill for me better than these other systems. I would guess that the FF RAS would be on par with the #45 SIR.
While they are not cheap, they do have more benefits for the buck than other companies similar setups in my opinion. If you set a rifle up with one, I dont think you'll be disappointed. Good Luck!