For me personally, it would be REAP > Zeus Pro 640 50mm > IR Hunter MKII 35mm
Zeus Pro vs. IR Hunter MKII 35mm:I would give a slight edge to the MKII on image quality and ease of use, however the Zeus Pro 640 has digital position-readouts for windage and elevation and the ability to save zeroes for multiple rifles or loads. Also, the Zeus Pro has a focusable objective lens, internal shutter, is recoil rated up to .50 BMG compared to .308 for the IR Hunter, it meets milspec standards for submersion, it has a 10-year warranty on the thermal sensor, it is lighter and more compact than the MKII and the video-output quality is better.
IR Hunter MKII 35mm vs. REAPThe IR Hunter MKII 35mm and the REAP-IR are basically identical in terms of image quality, magnification and field of view, detection range etc. Both units use the same BAE 12 micron 640X480 sensor and OLED display. They both use a 35mm objective lens providing 2.5X native magnification. Both units have quality quick detach mounts that will return to zero when removed and remounted on a rifle.
However, the REAP-IR has digital position read-outs for windage and elevation, the ability to store multiple zeroes, and a stadiametric rangefinder. The REAP-IR is smaller and lighter than the MKII 35mm (20.8oz vs. 34oz) and is more versatile (Helmet-mountable, handheld, weapon mountable) than the MKII.
REAP vs. Zeus Pro 640 50mmREAP has the edge in image quality and is smaller, lighter (20.8 oz vs. 32oz), more versatile, and is easier to use. The Zeus pro has a focusable objective lens, internal shutter, better warranty and environmental and durability ratings, and has better video output quality. To me, the performance, size, and versatility of the REAP outweigh the the advantages of the Zeus Pro.
Here are some comparison videos:
Zeus Pro 100mm vs. REAP
https://youtu.be/isOhP8KkzKwApollo Pro 50mm vs. REAP
Apollo Pro 50mm vs. REAP
(This is Rich from UNV BTW)