Currently owning and having owned all the major brands of thermal scopes for personal and professional use I will offer my personal thoughts:
IRD MK II/Reap-IR/IR Patrol all produced the best 640 class image of any system/brand I've personally used period. These models just are really sharp and provide awesome contrast. I think they have the best quality OLED screen which has to help on your eyes. Also great housings all feel and look high end construction wise. The negative for me is they are slow to power up and get a shot off 10+ seconds(pulsar like start up time). I leave mine powered on to counter this (powering up "booting up" eats more power than just leaving them on anyway).
Armasight Zeus 42mm/75mm are just pure hunting scopes. I really like the size/weight/construction/controls these are a proven design and have been out in the field for years now. Personally I like these scopes because they do everything you need and do it fast in field conditions. Power up to get shot off these are the fastest scopes I've owned last two years. Image wise they are in the upper class of scopes the ability to adjust contrast and sharpness help you get the best image in bad thermal conditions.
Pulsar scopes provide some of the best bang for the dollar but I'm not a fan of their large scope size myself.
ATN I really like the old THOR line owned a half dozen of them great work horses for me overall. That said the new HD models I'm not sold on yet mainly the cheap eye piece on the rear I'm not a fan of.
Overall a larger lens will always give you more detail, but at closer ranges and running game their narrow FOV can in return be a handicap. Personally in a 640 scope I'm a huge fan of the 2X scopes they have a bigger FOV and work great in timber plus provide 2X and then a good 4X zoom to shoot in open areas for a few hundred yards. Just a good do it all magnification. I had a Zeus 75mm , but it turned out to be more mag than I preferred for the wooded country around these parts so I sold it and kept the 42mm Zeus.
OLED vs LCD? Well I've yet to see a LCD in a thermal scope that looked as good to my eye as the OLED models. Feel free to argue away, but the LCD's I've seen just don't have the contrast the OLED's have.
Thermal scopes come in many models and lens sizes for a reason. There isn't a one size fits all for everyone. A lot of the guys here hunt larger open ares and perfer a big 75mm or 100mm lens. Some of us hunt more brushy/wooded country and perfer 35mm to 50mm lens to match that type terrain.
Same goes for size and weight, if you ride around in a truck or ATV maybe walking a few hundred yards to your stands. Well, weight/size isn't a major factor. If you walk miles spotting and stalking in a night carrying all your gear (scope/rifle/pack) then every ounce you can shave off your scope and gear turns into less pounds you have to carry all night.
Power up speed is important to me when a coyote steps out in the open and will disappear back into the timber in seconds. Hunting a large more open area with a lot of time to react, a few extra seconds power up not a huge deal.
My best advice is to match the features that matter most to you to the scope you buy.
Thanks for the input to Terry & everyone else. A question about terrain y'all hunt. Here in Yankee-land I usually call just inside timbers, waterways or narrow tree shelterbelts with open soybean/corn/pasture to my downwind side. Sometimes the coyote come in through the thick timber/cover but typically they are in an open field between 50 & 100 yds when I shoot em.
What is your terrain like where you guys hunt? I spent a little time near Quanah, TX & saw a variety of open pasture lands, thick, brushy ravines & steep rocky ground.
Where I'm headed with this is trying to get an idea of what size objective you guys use depending on the terrain you hunt? Oh, and Terry, if you have your choice of all the brands you've owned/used, which model scope do you find yourself reaching for the most?
Thanks again guys & have a Happy New Year!