Q-Core SLX Insulated

Exped Ultra all the way. I have the 5R, mummy, and it is perfect for a side (and rotisserie) sleeper. (I do not find the mummy shape a barrier to comfort and it’s a lot lighter than the rectangular model.) Big Agnes’s Rapide is also very comfortable — I have the Q-Core SLX, the Rapide’s predecessor — but it’s true that it feels colder than the R-value would suggest. The Exped in my experience feels accurately rated.  I use my little foam sit pad as a knee pillow, and be sure you have a high enough pillow, too. I slept comfortably this way for three months on my Southern Alps traverse. 

I think 2 pads is the way to go, but don't buy a summer pad and a winter pad. Instead, get a 3-season pad and a CCF pad—something like the Thermarest Z Lite or Nemo Switchback. They weigh under a pound, have a 2.0 R rating, and are relatively inexpensive (especially if you're willing to buy used). CCF pads are also handy because they can double as a durable, waterproof, insulated seat if you want to sit down for lunch out in the snow. I have a Big Agnes Q-Core SLX for 3-season backpacking. If I stack it with a Z-Lite, then the total R-value is 5.2, which is enough for any winter camping I'll ever do.

I have a big agnes q core slx that I paired with a diamond park bag from them. Granted both have been discontinued, but at 6'4 and around 315lbs as a side sleeper I had very little pain (hips and shoulders would occasionally touch the ground). Would see what BA has for current pads and find something similar size wise

We use an Agnes XL and love the Agnes mats. Super comfy. We also put a foam pad under it and use a sheet to keep it all together. The sheets are king sized and wrap around and under. We also do the matching sleeping bags and sometimes connect them together and other times we do not. My wife has a cot but anymore she leaves it at home and sleeps on the pads with me. Also....it helped us a lot when we got a bigger tent. I know you are talking one night and all ...but getting a bigger tent we could stand in and have a little floor space with rugs to walk on makes things much more cozy for us. Takes a little longer for setup and takedown ...but worth it for us.

Have you tried a 25"? I ask because I am 5'10, 325 pounds and have back problems and the BigAgnes XL pads have worked really well for me.