Things I would consider for Garmin models:
1) Screen size and controls. If it’s a 3″ or 4″ touchscreen is fine. If it’s smaller than that (like the eTrex series) then I would prefer the joystick or button controls because it’s just too small for fat fingers. If you want to use it frequently with gloves then you may prefer the joystick or buttons anyways.
2) Battery type. How long will you be away from power? Some have rechargeable battery packs, some us AA batteries. I find it a lot easier to carry spare AAs than bother with buying spare Garmin batteries. Some models have the option of using either so you can bring spare AAs if your rechargeable one dies.
3) Map support. You said you wanted to use onX so it has to have a color screen, that rules out the cheapest ones like the eTrex 10. I haven’t used the onX maps but I do have the Garmin HuntView maps which are similar. There is a *lot* of information displayed so this kind of comes back to a screen size question. I would consider 3″ that I have now the minimum but 4″ would be really nice. Some come with built in topo maps but they are the lower resolution 100k maps and are not routable. If you are going to buy a chip anyways then save money by not buying one with built in maps. For detailed maps you need to use a chip. One difference between Garmin 24k or Garmin Huntview and onX is that the Garmin ones will be routable so that you can get turn by turn directions on roads on trails, onX is not routable.