Not knowing what part of the country you hike in, I need to qualify this by saying that I hike mostly spring through fall in the lower Midwest (Ohio, Kentucky, and Indiana) - think high humidity and hot. Your neck of the woods might be different, and what I say may not apply to you.

As far as not wanting to sleep in a sauna, a double-walled tent won't automatically prevent that. Last summer, my brother and niece were on a trip with me; I was using a Hubba (which I highly recommend) and they were using a Hubba Hubba. It was a nice muggy August evening. They used the rainfly on their tent, and zipped the vestibules completely shut (I tried to persuade them otherwise; no luck - he never did listen to his big brother very well.) I didn't even pitch my rainfly. The next morning the inside of their tent was soaked - very sauna-like - while I had no problems.

My points are:
1) Button up a double-wall tent and you'll get the same sauna effect as a single-wall tent creates when it's buttoned up (and sometimes when it isn't.)
2) I also prefer a double wall tent, if the interior wall is nearly all mesh. With a double wall tent, I don't have to pitch the fly, and get great ventilation (in fact, with the Hubba, it's just like sleeping under the stars.) With a single wall tent, you have no choice about ventilation beyond whether to leave the door/vestibule open. You'll always have sidewalls pitched. (Some single-walls, like TarpTent, provide for generous amounts of ventilation that rival double-wall, non-mesh inner tents - but still can't hold a candle to the Hubba or other mesh-wall inners like the Seedhouse SL1, in my opinion.)