I've no experience with the Hubba HP but quite a bit with the Monoframe Diamond. As you've guessed, with all that nylon on the inner, it's a toasty little tent. It's been my go to shelter for cool to cold windy weather for the past four years. With stake and guy substitution, my carry weight is right at 2.5 lbs. The center height is 39", so there's plenty of room for most people to sit up. There is also enough room to store all my gear beside me in the tent. I'm 5'9" tall and have enough room to lie down without touching the ends. If you're over six feet, this may not be the tent for you because you may find your head or feet wedged into the ends of the Diamond. This tent has weathered some ferocious storms with no trouble. I wouldn't use it if I was anticipating heavy snow loads but otherwise it does great.
IMO, the large vent on the rainfly is an integral part of the tent. Keep in mind that you're venting moisture as well as warm air. If you're concerned with conserving as much warmth as possible, you can always zip the nylon panel on the inner fully closed. In practice, I've never done this, preferring to keep a good size opening at the top of the tent for ventilation. On occasion, I'll get a face full of spindrift, but prefer this to sleeping in stagnant air all night.
If you're interested in this tent you may have to act quickly because it appears that Montbell has replaced it with the Crescent 1 this year. Outwardly, the two tents seem similar, but the Crescent 1 is a double/single wall hybrid that uses a lot of mesh.