If you are choosing between just those two, I would go with the MXS. It is a good, solid detector that gets really good depth. When it comes to customer service, no one beats Whites.
If you are open to other detectors, I would look at the Nox. Construction wise, not quite as solid as the MXS, but, it is a multi-frequency detector that you can choose the one particular frequency or all of the frequencies. That is a feature neither the ATP nor the MXS offers, and that is a HUGE piece of tech. When my MXS dies, I will likely replace it with a Nox.