You could start as simple as a 556 front cap, and a $25 Precision Armament thread adapter to reduce the included 5/8-24 direct thread to 1/2-28 for ARs. The 556 front cap makes the biggest difference of all of them, and the included 46 front cap does well on 9mm and 308. Of those two, get the 9mm because it's only maybe 10 thousandths bigger than the 308.
As far as ASR vs Keymo the trade is Keymo is a little longer, a little heavier, and a little more expensive for a mount that requires almost no attention. ASR is prone to a few operator errors, but is shorter, lighter, cheaper, and has more available thread pitches. So if a 458 Socom or FAL or HK is in your line-up the ASR mounts are more available. But if your barrels only come in 5/8-24 or 1/2-28 that's a wash.