Aside from our rental, I don't have personal experience with any of these products. I know some trail groups use DR powerwagons with success.
I have a lot of time in gators - both 4x4 and 6x4 - and they work great. Fast, dependable, large capacity, but not really suitable as weapons in the singletrack quiver. A number of these power wheelbarrow devices might be. They are maneuverable, can climb grades, and are about the same width as handlebars (28ish inches, more or less) to fit between trees, and so could conceivably be used to move material right to where it's needed, even if that's a long way from an access road.
I don't know what to think about capacity. Advice on the web seems to be to buy big - as much capacity as you can. Yet on the other hand, we need to cross bridges that were designed for pedestrian traffic, not motor vehicles. Also, it would be very handy to just pull into the back of the Metro South trailer next to the grill and not have to mess with a larger, dedicated trailer or find someone with a pickup truck each time you need it. That means it needs to be pretty lightweight and compact.
Anyway, appreciate the comments and advice so far!