Smart move. There are lots to choose from. You know my answer, since I have the HH PI, but others use the Whites, Tesoros and Garretts with great success.
I don't think you will go wrong with any, as long as they are water proof and you put the time in. That's my problem...no time.
I think the Whites can be swung faster than the others. I do know the HH is a slow swing detector that does go deep and can be tuned because it has a frequency knob. Not sure about the Whites and Tesoro.
The Garrett I believe is the only PI that has ground balance out of the 4. It has a steeper learning curve. It also has the hi-lo, lo-hi tone and frequency knob. Multiple coils but I hear it's heavy. Probably a great under water and diving unit.
Not sure how deep you will be diving ( if at all ) and where you will be hunting, but the one thing the HH has over the others is it's super light. No need to buy different shaft configurations or hip mount it. And it also has the option of 2 coils.
Keep at it...I am sure whatever you get you will have fun with it...which is the bottom line. Just don't be too discouraged when your junk out numbers your goodies by a wide margin at first. That's the price of the PI...but you don't miss much.
JC