I don't think they're any different. However, keep in mind that the 800r has 8 channels of 192khz/24bit converters, impedance matching and a few other 'extras'. I think this is the reason for the price difference. But if in doubt, why don't you email mackie?
However, if you could use the extras of a mixer, go with it! Some shops also offer a modification for the Onyx mixers, making the direct-outs post-EQ which makes them available when recording.
Front End Audio does this, I'm not sure if there are more, you should look around for that. It's also possible to do this yourself, but this will void the warranty.
"Music is a treasure hunt. You dig and dig and sometimes you find something." - Ry Cooder (Buena Vista Social Club)