I find that midlayers are only good for light activity, such as camp chores, etc... & of course for added warmth under the primary insulation layer. Otherwise they're just too warm. While we all develop our personal preferences & priorities, I "budget" a proportionately high number of ounces toward my midlayer. It tends to be the "warm fuzzy" feelin' thing I wear around camp, at night, at rest stops, etc.
Having worn the powerstretches, cap 4s & 100wts, driclimes, & others I've finally settled on 250-350-ish g/m2 merino, preferably in a hoody. It's a much more adaptive layer than a synthetic insulated piece, allows me more flexibility & a great range of comfort. I've been using a Patagonia wool 4 hoody; mine weighs something like 13.5 ounces. I have similar hoodies from Ibex & Icebreaker that are a little heavier, but still in a reasonable range for my tastes.
As part of the above "defining layers" discussion, the piece I put on & pretty much don't take off until done is my baselayer. My midlayer is described above. Then I carry the insulation layer, which right now is a ~5oz down vest. Top it all off w/a hard shell. That combo'll take me through any 3-season stuff, happily.