The Equinox 800. It's SMF is going to perform the same as the AT Max in perhaps 45% of conditions and better than the AT Max in probably 45% of conditions. That being said, the AT Max has the following characteristic that, depending on your personal preferences, could make it "better" machine than the Equinox 800.
One, the AT Max is an S-shaft design. Some prefer these ergonomics over a straight shaft design like the Equinox.
Two, the AT Max uses AA batteries, whether lithium primaries, special lithium rechargeables, alkalines or NiMH cells.
Three, the AT Max is more reliable than the Equinox with its waterproofness.
Four, Garrett is an American company and the AT Max is made in the USA.
Five, the AT Max has a TON of aftermarket coil options available.
Six, the AT Max has the option of integrating wireless headphones and the Z-Lynk Garrett Carrot.
Not all of these will matter to you, but some might. And your situation could be different in that you're already familiar with the AT Pro. So the learning curve with the AT Max will be far easier than the Equinox 800.
I personally think the Equinox 800 is a better machine and will find you more stuff in more conditions/environments than the AT Max. I have the AT Max, and I really like it. I love the ergonomics, its design, its reliability, the batteries it uses, etc. But I basically got it used for $200. So is it worth paying...$400-$600 more to get a used Equinox 800? Absolutely not (in my opinion)! But if I could get the Equinox for $200, I'd sell my AT Max in a heartbeat.