That's not a whole lot of differences, but then, these projectors in most ways, are identical. The thing is, the first two items above make huge differences in overall performance. At the same price, there's no reason why anyone would choose the less expensive HC8100.
The "average" consumer looking for a projector is likely to be well satisfied by the HC8100 projector, but those of us who are enthusiasts, hobbyists, or just more into performance, would, if they got to compare the two on the same content, would likely to be shouting from the rooftops about how much difference the black levels of the 8500UB would make on those darker scenes found in most movies. They would also likely be shouting (though more quietly) about the benefits of CFI on sporting events, and even, on some movies. Too most of you who are first time projector owners, the Home Cinema 8100 should please you. Over time, as you become more critical (if you do) and seek more overall picture quality, you would quickly appreciate the significant jump in overall picture performance by going to the 8500UB.
Personally, I'm a huge fan, of the 8500UB, not just for the black levels though that is the key reason, but because of the blend of good best mode brightness, combined with much brighter than most, brightest mode performance. That combination is what you need for a projector that will work well for both movie viewing and HDTV and sports. I say to all who can here me. If it comes down to these two, and the budget allows - spend the extra, it puts you in a whole higher class of picture quality. I'd like the CFI, but that's not critical. I'm still getting by fine with neither of my own projectors offering CFI (JVC RS20, and the Epson 1080UB - that would be the 8500UB, but two generations earlier - the one before they added CFI.