The 800 handles surprisingly well. It's much like an overgrown 500/4 with a bit more heft to it. Once when you get to the 600, it's heavier by a bit.
IMO, the 800 is overpriced at nearly 12 grand when it was introduced. When I'm dealing with focal lengths that long, I actually prefer to take a longer optic in the field. 850mm and a 1040mm, but they're dedicated telescopes.
If I had to pick one, I'd go for the 500/4, but make sure you get a good batch. I've run into some that were DOGS and some that were outstanding.
And it all depends on how far you have to hike. 10 pounds of gear doesn't sound like much until you have it for hours and hours on end. The telescope setup that I have weighs in at a no so featherweight 17 pounds for the optical assembly only, but to me it's worth it in optical quality. If I HAVE to travel "lightweight", it's the 500mm. And when the lighting gets dim, I'll reach for the 500 due to the IS and the AF. But, when the target is close by and I don't have to hike, (and it's bright outside), I'll reach for the t-scope. All depends on the situation.