I went directly from a 860 to an 880.
If I had to make the choice all over again, I would definitely would go with the 880. A few reasons:
1. I think the deck itself has a classier look to it, the simple screen is very nice and mellow looking.
2. the 880 has a more solid feel to it, especially the knobs. I owned two 860's (still my second favorite deck), and the knobs felt sort of flimsy and cheap.
3. the 860's are known to have problems with the ground trace- basically, if it goes, youll have noise in your setup you cannot get rid of.
4. one of my favorite features- you can turn the internal amp off on the 880, that option wasnt available on the 860.
Theres a few other minor things, such as different display options (you can have the display show your voltage on the 880, I think that was available on some earlier models also).
Add in what has already been posted about the dual 16 band eq's and the slope adjustments (I wasnt aware of that), and the choice is pretty clear.
The only downside I can see to the 880 would be if youre more into the flashy looks- the 860 does offer the color screen. Other than that, Id say go for the 880, no doubt.