I haven't used the AT Max, but I do have an AT Pro. It's a great machine, especially as a first one. Not overly expensive and very capable.
The big advantages of the AT Max as I read about it seem to be the all metal mode and Z Lync built in with wireless headphones.
All metal mode will provide a constant hum that will get louder as the detector is over any metal. It should help you detect deeper objects, but not provide any information on what that object is. I have it on my Rutus Alter and, to be honest, so far I favor the descrimination modes. The Rutus has mixed mode with both descrimination and all metal that I use sometimes. I assume that's how the AT Max will work, but I don't know that for certain. I'm still learning the Rutus so I may get better with the all metal modes as I use them more. But so far I haven't seen much benefit from them.
As far as wireless goes the Z Lync is a nice addition. I have the orginal module for my AT Pro. It's nice not being connected to the detector. But if you buy it separately, it'll work on almost any detector with a 1/4 headphone jack. So one unit can be swapped to many different machines and if you get hooked in the hobby, at some point in the future you'll at least consider getting additional machines.
I expect the AT Max to be a great seller and very capable. But you won't lose alot if you go for the AT Pro now and add the Z-Lync system. At least in my opinion.