Hi Greg,
I have an MXT Pro ( with tones and back lighted screen) and the new AT Pro and the machines both will do an excellent job on the dry land, I puchased the AT to get the waterproof factor so I can hunt in the rain. and so far this detector has lived up to most of the hype. I have not taken mine to the beach yet, but had a chance to take it to a really junky lot shortly after a good rain and ended the day with a few old coins, some clad and a couple good relics, but I did notice that with the targets being damp from the rain it tended to make it harder for the AT's iron mask/roll feature to do it's job, but it still performed very well. I really only had 2 complaints with the AT and those are, 1. headphone cable is a little short, but I'm sure that can be remedied, and, 2. I really wish it had a back light on the screen so it could be used to night hunt. Other than those two items I don't know of any other real issues with this machine.
I would say that if you buy an AT as a back up to your MXT that you will just need to decide which machine is actually the back up, The AT Pro or the MXT, But I will keep both as they both have a great array of great features for the type of hunting I do.
Hope this helps a little.