Not sure how much experiance you have on beach detecting Bluearmy, so for give me , but a PI is ok on the wet sand ie highly mineralised sand, you will dig deep but be prepared to dig lots of iron, untill you you master the PI...not an ideal machine for the dry sand areas either.
As for the AT pro may be a better choice especially dry sand, you will not get the best depth on the wet sand all though I'm sure others may say otherwise.
If as you say you want a machine solely for the beach there is only really one make that masters the beach...Minelabs all ways have been and probably always will be.
Try a Soverign and pop a 15" WOT coil on after you have mastered a smaller coil pack coil, or an Explorer or E Tract, all capable of pulling out deep coins and Bling on any beach even junky areas.
I am confident serious beach searchers on DW will agree and there is enough of them on our forum.
What ever you decide keep us updated on your final choice and finds....Spikey