Whatever you get, make sure you can get rods for it. You WILL break them. Guaranteed. I jinxed myself earlier this year, went two seasons and no busted poles on my two man then disaster struck on New Years. I stayed out late, and the combination of frost, beverage, impatience, and cold resulted in two snapped rods and a lot of swearing. The next day I bought a flip over.
I think you're on the right track on buying big - it doesn't make sense to get a small one. Eskimo makes a good shelter and have parts available with no problems. Clam had issues with the material, but on the lower end models. The quilted material is better quality but crazy expensive for what it is. The Shappell's have good reviews as well. insulated fabric is heavy, so remember that when deciding how you're moving around, most popups double in weight from a standard 300D cover.
I think the insulated popups are nice if not a little overkill. If you have the money to spend on them, go for it - but I'd be fine with a regular popup and a good heater (which it sounds like you have). In my 2 man (60x60) a regular single panel Buddy style heater (mine's a Dynaglo) will cook me out of it if it's higher than low. I only turn it on after sun down, and I've yet to turn it on in my flip over.
Glen's Army Navy if they still have stock usually have great pricing and free shipping, same with Reeds/RedRock.
Lot's of happy folks with Eskimos.... just sayin'.