If you are only going to have one beach detector I would go for a VLF, probably a Minelab Sovereign or Excalibur. The biggest difference is the Excalibur is waterproof while the Sovereign is not.
If your current detector can do a decent job at the beach (i.e. isn't falsing all the time), or you can get two detectors, then you probably want a White's Dual PI. I strongly recommend against a PI as your only beach detector unless you are positive you will only visit iron free beaches. I find that digging dozens and dozens of nails on an iron infested beach counteracts any of the benefits the PI would bring to the table. But, on a beach where there is little or no iron junk you will be mostly digging good targets and the extra depth will make up for the loss of discrimination.