Which EE quilt to layer under a MLD spirit?

You should talk to Tim, I reckon your spirit will add 20f to another quilts rating.
If you got a 30F EE it would be good to 10f together.

I am a restless side sleeper also and went for a wide sleeping bag.

You are in a very similar situation to me, except opposite. I own a 20 degree Revelation and I'm currently looking for a lightweight synthetic quilt (either Spirit or Prodigy) to layer over it in the winter and to also use alone in the summer. I am trying to choose between a 40 and 50 degree option, currently leaning slightly more toward 40.

I honestly think whichever temp rating you choose will work out fine. If you are mostly going to be experiencing temperatures in the 20-30 degree range, the 10 degree Enigma will be plenty warm for those conditions. I've taken my Revelation into the 20s with no problem. I can't speak to anything colder than that, but I imagine that if you layer your Spirit quilt with it, it should be able to go down to 0 degrees. Having a 0 degree Enigma might be more than you need if you're mostly going to be in the 20s.

As for the regular vs wide option — I'm almost identical to your dimensions (6'1 and 150), also a cold side sleeper. My Revelation is both regular length and width and it works well, although sometimes I wish I had the wide version as cold drafts can come in when I roll over in the night.

Hope I was able to help some.

Hey rm. Official EE recommendations would be:

First, for temp, if you tend to sleep cold we normally suggest getting something rated to 10° lower than you're likely to actually encounter, but with the good options for insulated clothes, you could probably go with the 20° rating for 20° trips. That layered with the MLD will give you some added buffer on those colder weather trips as well.

For sizing, the Long shouldn't be necessary unless you normally stash extra clothes/gear in your footbox to keep it warm overnight. Esp. since moisture accumulation is a concern, you shouldn't plan on ever covering your head with it as moisture from your breath will compound that problem a lot faster. Also, if when side sleeping you normally curl your legs a little (I do, and I think most side sleepers do), then the Regular length quilt would still provide ample space (I'm 6'2 or 6'3" and use a Regular length, except on super cold trips since I do like to stash spare clothes in the footbox).

For width, just on body type we'd recommend the Regular, but it sounds like you'll find it an easier transition to go with a Wide, so either would be fine.

10F R/W EE enigma + 38F XL MLD spirit + thermarest xtherm, on snow – temperature inside my mid was 20F with light inside breeze. Predictably the EE+MLD quilt was more than enough warm enough. Sizing was nice enough. The number of straps was slightly annoying, but quite bearable. Did feel a bit cold through the xtherm (not sure if the inside of the mat has sustained some damage). Will layer a thin closed cell foam over the top next time. Both are very nice quilts. Glad I got them :). Will be a while before I can get somewhere colder, I'd be very interested to see how I go in 0F.

I find it much warmer to put my quilt inside my sleeping bag. I am 5'11" and 155. I have a 6'3" bag that is 15* rated and 900 fill 61" width. Without the quilt, it's good down to 25* for me. With the 15 ounce quilt inside filling all the open space, I can get to 5*. EE now has a sleeping bag called the "Convert". I'm willing to bet the 20* version would suit your needs.