Either will work with a #40 and Eotech.
TN, if you really want a FF RAS (and I dont blame you...) you CAN do it. It just wont be a do it yourself kind of deal. If you want, you can think about a muzzle brake upgrade as well. Here's what you can do...
Get the FF RAS. Send it and your upper to Kurts Kustom Fireamrs. He will take you old muzzle brake off, install the FF RAS, and put whatever muzzle brake you want back on legally. If you consider this, look into Kurts A2 or A3 muzzle brake. They're pretty good brakes as far as brakes go, and they look better than anything on the market. I know this isnt the quickest or easiest way to go, but I just dont like for folks to think they CANT do what they want.
If you;d rather do it yourself, Coldblue gave sound advice. Get a RAS II or a SIR and you will be able to do it all yourself without any modifications or real work at all.