I have an upper in both calibers. It's a tough call for the application described. Both have their +/-.
Here is my input based on MY experience only:

The 300 has slightly less recoil and bark than the 7.62x39 in the 127/147 gr I shoot. It has notably more than the 5.56 but it's not bad. I don't know if your clients would be OK with it.

The 300 can be more expensive to feed (as compared to the 5.56 and 7.62). This can inhibit regular proficiency/practice.

The 300 uses EVERYTHING from the 5.56 parts bin but the barrel. This makes it cheap to build and allows conversion to 5.56 if the experiment doesn't work out.


The 7.62x39 enjoys cheap and common ammo.
Can take some work to get them to feed 100%. 7.62 Ar's have a reliability reputation for a reason.
It can be finicky with mags . Especially as those those mags get higher in capacity. Mags are unique and require an investment . (FWIW, I read that only ASC mags should be counted on. I use them in 10 rnd only).
Soft point / steel cased ammo should be avoided for reliability. You need to find ammo it likes and buy allot of (and only) it.
More recoil than the 300 and 5.56 in my experience.


Personally, I would stick with the 5.65 flavor and be done with it. Low recoil, cheap to feed, ammo is everywhere, flat shooting, reliable and plenty powerful to stop most critters of man sized and below.

__________________
Dave