The extra tweaking on the 800 is actually a time waster, a distraction. I can appreciate the 2nd user slot, but I don't use it myself. In my 8-plus years of experience with the Whites V3i, I was commented to by more than one veteran detectorist, "You waste a lot of time tweaking."
So, in my personal experiences, ""Aside from the wireless headphones, are the 20khz and 50khz frequencies worth the price difference?"
Unequivocally, no it isn't worth the price difference if you don't detect for gold nuggets and flakes. It is just the opposite. I grab my 600 just as quickly as I grab my 800. jm2c