In terms of tone and headroom, using it as a simple combo (no extension cab), there is very little real world difference. You're talking about a 350 W amp vs. a 400 W amp in the same series (meaning basically the same basic tonal controls).
The 800 is 7.5 pounds lighter, which is a lot in terms of percentages. However, the 500 is not even a little bit burdensome in the weight department, at a mere 36.5 pounds. It's an easy one-hander, even when walking several blocks. The 1X15 combo amp that I had for 30 years as my main bass amp has a few pounds shy of twice that weight, and not even 30 percent the power – not to mention lacking most of the modern, and extremely useful, features of the Rumble series (XLR line out with ground lift, headphone jack, aux. in).
The 800 has a pre/post DI switch. Potentially useful...or not. (I have never had a use for this; never a single issue with basic post-EQ DI.)
The 800 lets you use a wider variety of extension cabinets (4 or 8 ohm, as opposed to just 8 ohm with the 500). Again, potentially useful, if you have a lot of cabs, or want to be able to use a wider variety of backline cabs. But also not at all useful to a lot of people, and irrelevant if you aren't even considering an extension cab.
Point being, in the grand scheme of things, the 500 is not lacking. It's incredibly powerful, light, and full featured...however slightly much better the 800 might be in these areas.
The 800 is the "better" amp on paper, but a majority of people would appreciate those differences pretty infrequently, I'd think. You'd have to be playing through extension cab most of the time, make good use of of pre-EQ DI, and potentially be plugging in to a variety of backline extension cabs (or just own and like using a variety of extension cabs). The weight differences are irrelevant IMO. They're both extremely lightweight amps to me.
To my thinking, the second you know that you almost always will not be playing through an extension cab, I think the answer easily becomes "500." And if you know you won't be hooking up an XLR cable in addition to that, then that is a definite "500." $210 for advantages you'll never really use? Not worth it, IMO. That money is better spent on the Rumbe 500 slip cover, an extra power cable or two, some nice cables, and, if you like it, the dealer exclusive white tolex version ($36 more expensive).