Tony, I have both. The 8500 has the ability to adjust the sound volume in every different notch category, not just iron volume. You can notch out or silence any category including dime and quarter. It has a fast mode and 3 others with different filters. These modes, to me, seem like they affect the all metal mode associated with it. The fast mode seems like is a quicker retune, etc. It offers another 4 tone mode which does not modulate the sound from shallow to deep. You can set an automatic offset in the ground balance. The threshold and sensitivity controls are independent of each other. You can press and hold the menu button to switch quickly from disc mode to all metal. Release and you are back to disc mode again. Also if the detector is not sure which notch category the target is in, it will give a black indicator above the most likely category and a gray indicator over the less likely ones. Some think the 8500 is deeper, I really can't tell. Oh, almost forgot, the 8500 has a backlight which I have never used.
Some people complain that the sound is "clipped" more in the 8500. The tone is definitely shorter in the fast mode and the clipped part doesn't bother me at all. If you want a super fast response, it must be short.
Both are very capable machines. Which is best is pretty subjective. I like the 8500 best but many very capable detectorists favor the 8000.