<<what is the difference with the Zeus and Zeus Pro>>
I've owned both ...
The Zeus 75mm, 3x, 640(30) is lighter and smaller.
The Zeus pro 100mm, 4x 640(60) has a set of fancier reticles that provide some support for longer distance shooting. And it is rated for .50BMG recoil, which the Zeus Std is not.
I think I had one of the "earlier" ZP 100s and even though the software was upgraded to the latest and greatest, I'd say the Zeus Std had a clearer image. Armasight told me that was because of the 60hz. So they said the 60hz version of the ZP 100 was actually less clear than the 30hz version. Well I had the 60hz version.
So, anyway, based on my experience with them, for hunting, I would rather take the 75mm 3x, because it had a clearer image AND because it has less magnification. The primary reason I sold the ZP 100 was because 4x magnification was too much magnification for me when hunting. One night was I out there with 2 coyotes one to the West and one to the East ... I could hear them within 200yds, but I was swinging the gun around trying to get them in my sights and by the time I did they were headed away ... then I think if I was in a field of multiple sounders swinging the gun around looking for the runners after the first volley, I might not see one running in on me. So for hunting, I would much rather have the 35mm Mk3 or equivalent ... 3x magnification max for hunting for me. The vast majority of my shots have been under 100yds, at that distance 1x is plenty of magnification, 2x might be perfect, 3x is the max.
Now in open country, like I've been in around Wichita Falls, a lot of magnification is useful for long distance spotting. So the M250XR would be great as a spotter. But I'd rather have less magnification on the gun scope unless I was mostly shooting over 100yds. So I see the 60mm Mk3 as carrying the LR spotter on the gun. But if I was going to do that, I'd want at least one other team mate to have less magnification to improve team chances to get any hogs running in.
The ZP100 is good for LR target shooting, say out to 500yds on heated steel, using the "mil-box" reticle. The Zeus Std can do that, but holding for wind with the Zeus std has to be done by judging off the target.
Comparing ZP100 to Mk3 35mm ... once we saw 40+ hogs at 1200yds heading in a line ... and we switched guns back and forth to compare the images and it was the same ... both devices were on black hot and both were on 2x digital. So effectively the ZP 100 was 8x and the Mk3 35mm was 5x, but as to clarity, the images were the same. This was May around Wichita Falls, so not crazy humid.
My Mk3 35mm is my "go to" scope for night hunting.
I've thought a lot about "upgrading" to a 60mm ... that would provide a better LR spotting capability ... and I might do it one day ... but for my use cases it still seems like having wider FOV trumps having more magnification.
If price is an issue, get a mk2 35mm, or even a mk2 19mm. You can upgrade either to the mk3 35mm or the mk3 60mm. You can get in to the BAE core and spread out your spending by upgrading later. They all have the same core and display.