Not to outright disagree with you michealnc, but I've not experienced any of the "weaknesses" that you've described. And, w/o more speciffic information like the frequency and coil configuration that you're comparing, I ain't buying the AT out performing the XT.
The Xterra properly set up will out do an AT anywhere that the control box is out of the water. Frequency choice, coil selection, and a host of other features make the XT more flexable/adaptable than any other machine in or under it's pricepoint.
If you want to dunk your machine or hunt in a downpour, get an AT. Otherwise get something else. You get what you pay for. So if part of what you're paying for with an AT is it's being waterproof, then how much technological advantage do you really think it can have?
And don't get me wrong! For what the AT was designed for, it's certainly fine. But since I can have more than one machine, if I want to go into the water, I'll look at one of the Fisher's or a Tiger Shark to compliment my land detectors.