The nox: has a better housing, better warranty, the image doesn't degrade as much in poor thermal conditions, more "realistic" image (helps with depth perception), weapons rated, true 1x optical thermal so that you can use it on a bridge mount with a PVS 14, and US made. I have had both units, and the still have the Nox.... the MH25 has since been sold.
The MH25: does have a smaller build/lighter (at a cost to the housing durability; there have been quite a few reports of people breaking them), better display (1280 HD as opposed to 640 on the Nox, which equates to a better digital zoom). The housing feels cheap and the factory eye piece is a POS (so you'll need to spend another $350 upgrading it to the PVS 14 eye piece), not weapons rated, and is slightly optically magnified (which makes running this on a bridge very difficult). The refresh rate is 50 Hz on the MH25 as opposed to 60Hz on the Nox and it is VERY noticeable when you're trying to navigate with the units. The nox gives you a smoother transition when on the move, the MH25 not so much. Also, in big open fields (like in my AO), on blk hot the MH25 over saturates the upper right hand corner with contrast making for an uneven "messy" image that actually affects your detection range(white hot doesn't have this issue though, but I mostly use black hot) The Nox evenly contrasts the entire area giving a greater sense of uniformity and extending the range to which you can see (I can see over 30 miles with the Nox easily).
This being said, the MH25 out performed the Halo 25 and Halo LR 50 units I had at the time in inclement weather, but does NOT outperform the Nox. The MH25, since it's slightly magnified, will do better as a handheld spotter for PID....but is not weapons rated like the Nox like stated above (nor would I trust it for that task, the housing doesn't feel built for recoil). If you are going to helmet mount it, that optical magnification will cause issues with depth perception and using it on a bridge (a PVS 14 is a true 1x, the MH25 is more like a 1.5 times even though it states 1.1x optical mag).
My hunting buddy has an MH25 and I have the Nox now, so I got to do comparisons of both using them as handhelds and helmet mounted units in varying terrain in all weather conditions, and the Nox holds an edge in general usage.
All together having both, the Nox is by far the better option. It's a more....complete thermal thats built for hard use. I bought one of the first MH25's in the country before the Nox was avaliable, but if the Nox was produced at that time I would have went with it over the MH25 plain and simple. Oh, and both can take rechargeable batteries (for completion sake).
