I have an AT Pro ($659.00 if I remember correctly) and a Minelab CTX (As DOD said, you don't want to know price). If you are the type of person who digs all signals, I would recommend the AT Pro. I don't know much about Whites in general, but the advantage of staying with the same manufacturer is that you understand the technology. I also bought the NEL storm coil for the AT Pro and now I find pending where I go deciding which detector to use. I do love the CTX, but sometimes the AT Pro works as good as or better (with NEL coil).
The bonus about the AT Pro is that it is Waterproof to 10 feet (standard headphones that come with unit are not).
Only downfall of AT Pro and that is due to the lack of patience I have is how batteries go in/out and also threading the cables into the unit. Houdini would even have a little trouble with it.