After using the 600 for a bit over a year now, I feel somewhat qualified to have an opinion on this.
I upgraded from a 1995? White's Eagle Spectrum, for a few specific reasons. Generally "new tech", Li-ion rechargable battery, waterproof, and wireless headphones. If not for those reasons, I was happy with the White's.
Yes, it has more usable individual frequencies, and gokd mode, but that is no interest to me. I hunt ghost towns, homesteads, and cellar holes.
At the time I made my choice, the differences between the two didn't really matter much. Since then, having the one instruction book that covers both models, and specifically learning the differences, I would choose the 800......Here's why.
The additional audio hardware. I rarely use headphones, but the 800 comes with a set of wireless. Also comes with a wireless module I could use any wired ones with. That pretty much covers most of the monetary difference.
The backlight.....The 600 has one brightness setting. I dont really care, but occasionally I would turn it down if i could.
Target tones......again, I don't really care too much, but If I could, I would make 3 tones, ferrous, non ferrous good, and nonferrous bad. The 600 won't do that
Thats it.....thats all the reasons I have. There may be ofther differences, but I don't even care enough to know what they are.
I have no regrets after getting the 600, and I won't be getting the 800, but If I were to buy again, I would get the 800.