Pac boots are great for warmth in really cold weather, but not intended for hiking. There's not enough ankle support for any real hiking and they generally weigh a lot. There's not much to learn about them that isn't said in product descriptions. I've always had Sorels, but there's plenty of good brands out there. My one suggestion is to make sure you don't get them too small. It's very very important you have good circulation. Having too small a boot or wearing too many socks or too heavy of socks leads to more cold feet than anything else. Circulation is key. I bet that was part of the issue with your current boots...too tight. I've actually taken socks off to give my feet more room and felt significantly warmer. It was totally counter intuitive, but works. So when you buy a pair, bring the socks you would wear in the coldest conditions. The boot should be comfortable, but not snug, just covering your foot without any constriction. Don't worry if they seem rediculously big. You don't want big air gaps in the boot either though. If undecided on fit, error on the side of having a little too much room than not enough.
I've always had leather uppers which I treat with SnowSeal to keep them waterproof. I never tried the nylon uppers, so I don't have an opinion. Nylon uppers might be a little more noisy if you stalk into and out of your hunting stand, where leather is quiet. I often do late season archery hunts in snow, so noise is a factor for me.
Most pac boots you can take the liner out. That's important because it makes drying them out much easier. I would never buy ones where I couldn't take the liner out. You never want to go out with wet boots, even if damp from a long hike out of a stand where your feet sweated. Need to stay dry at all times.
They come in different heights. My last pair were 12-14 inches high. I thought they would be warmer or help stay dry if crossing creeks. Not sure it helps too much, other than add weight. When these wear out I might go back to a standard height.
They come in different temp rating. Assuming you're not hiking far, I.e. only from truck to hunting stand and back, why not go the warmest you can get. If it's not super cold, then you'll probably wear different boots anyway. The longer you can sit, the more likely the deer are on the move....either from other guys pushing them from being cold and walking or the deer themselves getting more active to stay warm. I've shot a lot of deer from 10am to noon because other guys start walking.
Also....dropping a chemical hand warmer in the bottom of each boot once you get to the hunting stand makes a big difference. The small amount of heat goes a long way. Lastly, if you want extra protection, try the boot shields....those insulated boot covers you put on once you get to the stand. They help with wind chill. I can sit in the stand all day in nasty weather and cold temps without issue. But it took a lot of trial and error.