I've owned both models a few times and enjoyed both.
If you want one of the best swinging water machines out, that has to be the best salt water machine value, and well balanced, "deep" seekers, the PI Pro, is the way to go. I believe, it's rated to 200 ft. You'll find out shortly if you truely enjoy this hobby, because you'll be digging the smallest pieces of iron/bobbie pins in your area, but there will be no doubt in your mind, that you left any gold, by choosing, not to dig.
If the Pro is the one you choose, try and determine if the majority of your hunting will only be wading and wet sand hunting. If it is, go with the lighter black coil. If you'll be diving and waist to neck hunting, go with the diver/weighted yellow coil.
The BHID retails about $100 more than the PI and is a decent discriminater, which works decent in salt enviorments, and also swings nicely. The red, green, and yellow LED's are fun to watch(exspecially at night), but hard to see, when rear mounted. That combined with tone ID, makes it much more for the masses. It will find reasonably small gold jewlery. It's not a CZ20, Excalibur, etc., but it's not in the same price range either. It's only rated to 25ft. submerging, not a problem with 99.99999999% of the hunting world. The coil floating quirk's can be solved, by weighing the rod with shot or sand.
Both machines have to have the batteries charged by opening the case, which is not a plus for a water machine, in my opinion. Everytime this has to be done, there's a chance for it not to seal correctly. It only has to leak once, and you out "mucho" money. The Excal and Garrett Sea Hunter/Infinium, have the best recharge systems, neither have to be opened for recharging.
As with all machines, you have to get out and swing, you won't find gold in your recliner. HH