I'm not the original poster, but can chime in to say I still use both even though I took to the Nox right away. I'll take both to some hunts. Last weekend I bounced around to a few new spots using just the AT Pro and the 5x8 coil.
If you're good with the AT Pro in Pro Zero mode and hunt dirt, then you aren't missing a whole lot. (I did run into an area where the AT Pro was completely overwhelmed by high levels of aluminum and could just bounce numbers and occasionally give a beep. The Nox cut through it and I was digging coins. I credit that to the multi frequency working.) If Pro Zero mode doesn't speak to you, and you hunt mostly in standard mode, then default Park 1 on the Nox is a major step up.
Yes, I've come behind the AT Pro with the Nox and found more items, and other times I've found nothing additional at all. As an experiment, I thoroughly detected a 20x20 area within a property containing lots of pull tabs and coins first with the Nox 800 in Park 1 default, and then with the AtPro in Pro Zero. I found a few more coins, but only because the AT Pro had me digging some sketchier targets and a few turned out to be coins. (I could have changed the settings on the Nox to do the same.)
The AT Pro is plenty of detector and is hardly obsolete....until you have the time and desire to squeeze all of the coins out of a site. If I had to sell one, it would be the AT Pro, but only because the Nox is more versatile. I can find plenty with either. If I had more permissions than time, then either one would be great since you're just cherry picking anyway.
I have come to prefer the way the Nox tones sound, but mostly because they don't aggravate the ringing in my ears. I think the AT Pro tones are more descriptive, but they're also very loud and abrupt. (Crank up the volume to hear deep stuff due to the proportional audio and you'll blast your ears out if you pass over a shallow aluminum can.) The Nox 800 sounds like a video game, but makes up for it (and then some) by giving you the option of changing the volume and pitch of specific tones. Just being able to lower the volume of certain tones without completely notching them out is a big deal to me.
