Start with the sensor, not the screen. The number that decides how much detail a thermal can resolve is the sensor resolution. 256×192 and similar sensors are entry level, 384×288 is the common mid-range step, and 640×512 is where premium units start. More sensor pixels on the animal is what lets you tell a coyote from a fox, or a doe from a buck, at longer range. Screen resolution and recorded file size tell you much less. The T150's listing, for example, uses AI processing to make its 256×192 sensor look like a 384×288 on the display. That can clean up edges, but it cannot add detail the sensor never captured.
NETD is the sensitivity figure. It measures the smallest temperature difference the sensor can separate, in millikelvin (mK), and lower is better. It matters most on the nights thermal finds hardest: humid air, drizzle, fog, and the hour after sunset when the ground is still warm and animals blend into it. The T310 lists 25mK, the T150 under 40mK and the WG-T009 under 50mK. On a cold, dry night in November all three will show a deer clearly. On a muggy September evening in the South, the lower number is the one you will notice.
Field of view and magnification pull in opposite directions. A short lens gives a wide view for scanning, which is what you want when you sweep a whole field from a stand. A longer lens narrows the view but puts more pixels on a distant animal. The T310 lists a very wide 35° field of view from a 9mm lens, the WG-T009 lists 17.9° × 13.2°, and the T150 lists 11.6° × 11° from a 19mm lens. Digital zoom on any of them enlarges the pixels you already have. Use it to take a closer look at a shape, not to identify an animal at twice the distance.
Detection range is not identification range. When a maker lists 500 m or 1,498 m, that is the distance at which the sensor can register a warm object as something other than background, in good conditions. Recognizing it as a deer, and then identifying it well enough to make a decision, happens much closer. With a 256-class sensor, expect to know that something is out there long before you can say what it is. Plan your setup, and your expectations, around identification.
Refresh rate, listed in hertz, is how many times per second the image updates. At 50Hz or 60Hz the picture stays smooth when you pan across a field or a coyote trots through the frame; slower 9Hz units stutter and smear. All three monoculars here run at 50Hz or above, so none of them is held back on this point.
Thermal is one of the best tools for recovering a hit deer. The animal, its bed and fresh blood all hold heat for a while, and a monocular lets you check a thicket before you walk into it. Heat fades from blood and tracks fairly quickly, so start as soon as it is sensible to take up the trail. Remember that thermal cannot see through dense brush or glass, so a deer lying behind thick cover can stay hidden even at close range. Whether you may use a thermal device to recover game, and at what hours, is decided by your state, so check before you go.
Plan for battery life and weather. Built-in lithium batteries lose runtime in the cold, and all three models charge over USB-C, so a small power bank in an inside pocket is cheap insurance. Listed runtimes are about 10 hours for the T150, about 5 hours for the T310 and 4–5 hours for the WG-T009. For long sits in rain or snow, the WG-T009's IP66 rating is the most specific weather protection listed among the three.
A monocular is not a riflescope. None of these three is built to mount on a rifle as a sight. Hunters who shoot at night where it is legal still tend to keep a handheld thermal for scanning, because sweeping a field with a riflescope means pointing a rifle at everything you look at. A monocular keeps observation and shooting separate, which is safer and easier on your arms over a long night.
Know the rules before you go out. Thermal and digital night vision devices are generally legal for civilians to own in the US, but using them while hunting is regulated state by state and species by species. Many states allow night hunting of feral hogs or predators under specific rules, while night hunting of deer is widely prohibited, and some states restrict thermal devices or artificial light for scouting or recovering game in certain seasons. Rules on permits, landowner permission and devices vary and change, so check your state wildlife agency's current regulations before you take a thermal device into the field. This is general information, not legal advice.
Which one should you buy? For the longest listed battery life, Wi-Fi recording and the lowest price here, choose the T150. For the best listed sensitivity and a wide scanning view in the lightest body, with infrared night vision and a daytime color camera built in, choose the T310. If you want the toughest build, a distance estimate and a laser pointer to show a hunting partner where to look, carry the WG-T009. Pair any of them with a digital night vision binocular if you need to identify what you have found at longer range.