For freshwater only, it's the Tiger Shark hands down due to excellent discrimination. Saltwater it is a toss up in my opinion. The PI is deeper which is good in the sifting sands of a coast, but due to almost no discrimination of a PI, you may wear yourself out digging giant holes for wire bread wrappers and nails. The Tiger Shark doesn't have this problem because it discriminates out iron well, but it is not as deep.
There is a third option in the Minelab Excalibur 1000. It is more expensive than either but is about as deep as a PI and more important, will pick up a gold ring under a iron target which it will ignore. The Excal doesn't need to be ground balanced either so it is good all over without having to adjust GB.
Tony will give you a good deal.
Sandman