I have a 20 degree RevelationX, which I loved until I saw the Enigma :) In the summer months in the Sierras I have not generally needed to sleep in my hooded Stoic Hadron. So in the heart of the summer, a 30 degree Enigma would probably work well even if there is a cold snap (just add the Hadron). In the shoulder seasons, however, I have been very happy to have the 20 degree quilt rather than a lighter, but less warm quilt.
So what to do for 3-season use in the Sierras? Tough call. I personally would probably choose the 20 degree Enigma. It is still ridiculously light and only weighs 2.5 ozs more than the 30 degree Enigma. And even with the sewn footbox I think it can be reasonably vented during warmer weather. But my trips tend to be 3 to 4 days, where I am not as worried about a few ounces.
Either way the Enigma looks fantastic.