I had both for a while and a Kowa 554. I did some fairly objective comparisons and included the link that discusses them (thread was about 18x binos but have some small parts about the spotter too). It has low light and edge to edge clarity comparisons in the link.
Also attached is a resolution chart comparing the three. Obviously their zooms are all a bit different but you’ll get a feel for how they compare. Charts at 400yds in overcast skies.
General consensus is I like all three and they each have trade offs.
Weight. Kowa 8oz lighter than GR which is 8 oz lighter than Razor.
Length. Kowa is shortest. Close to GR (fatter though). Longest is Razor (only by a few inches).
Glass. The Kowa is the best. But has limited FOV and eye relief. The GR is about the same as razor IMO. The eye relief in GR is massive.
Ergonomics. I really like the finger tune focus (Kowa and Razor). I think the focus on the GR is stiff and tough to view and focus at the same time.
BL: I stuck with the razor. I like the focus the best. I got the LER 22x lens for it which was cheap and works well. And I got it for $575. Compared to $1000 for the GR and $1400 for kowa. All three fit in my pack and I didn’t mind having an extra pound for the weight for the cost savings. I also like the 48x zoom. According to my chart with my eyes it did matter. Also in edge to edge the sample I had was nearly perfect. I am happy with it.
Many will say the GR is arguably one of the most well balanced spotters out there though. Another one Rokslide folks throw out a lot is Athlon Ares 15-45x65. I’d get the first gen on that one though because it’s more compact and lighter.
Folks Doesn’t seem like too many have had hands on the UHD and Maven 18xs together yet. Had the chance to demo so will give it my best shot at comparing. That said I will draft a couple initial posts and then encourage folks that have specific questions to reply and I will do my best to get...
UHD Glass: Extra-low dispersion glass gives you an image with little or no chromatic fringe so the final result brings an ultimate clearest and sharpest image to your eyes
Interchangeable: Eyepiece Interchangeable Eyepiece allows you to swap the standard variable power eyepiece with a fixed 24 power eyepiece with a MIL ranging reticle in it.
ESP Dielectric Coating: ESP Dielectric Coating is a multi layer prism coating that reflects over 99% of the light to your eyes bringing you a clear, bright image that displays accurate color reproduction.
Aluminum Alloy Chassis: Aluminum alloy chassis offers you the strength of a metal chassis and the life time durability.
XPL Coating: XPL Coating gives you an extra protection on the exterior lenses from dirt, oil and scratches
Bak4 Prism: Bak-4 glass prisms reflect more light to your eyes thus will give you brighter and sharper image.
Advanced Fully Multi-Coated: Advanced Fully Multi-Coated lenses bring you better light transmission to bring optimum brightness and true color across the entire light spectrum.
Argon Purged: Argon Purging uses the inertia gas with bigger size molecules to purge any moisture out of the tube giving you better waterproofing and thermal stability .
Waterproof: Waterproof to protect the binocular in the harshest weather conditions or if accidently submerged underwater
Rotating Ring: Rotating Ring allows you to rotate the scope around tripod supporting ring into the most comfortable and convenient angle for observation