Are you going to SCUBA with it? Just curious. If not, then read up on the ATX; it's a little bit more expensive than the SHII (Sea Hunter II) but it looks really good from the reviews I read. If considering the SHII, I would also look at a used Infinium. They are out of production but a dandy machine although probably not so much for a beginner. Don't remember a lot about it but did choose the Infinium LS (ILS) over the SHII when I was getting a PI a few years back. The ILS seemed more versatile although has a steep learning curve.
The other one would be the AT Max. Again, I've only read the reviews but it looks pretty good on paper and has a few extra better features from the ATP. I have an ATP and like it (and would like to have the ATM, just can't justify it right now.) With the ATM being the new and improved AT, I can only wonder... there's a bit of discussion about it on the AT forum.
If you are planning to SCUBA with it, then your only choice between the two is SHII. The ATP is only good to 10 feet or so. Basically it's dunk proof if you trip and drop it in shallow water. Easy to clean up too.
Hope I didn't confuse you more...