Comparing a VHF and a PLB as rescue tools is a bit like comparing apples and oranges. The VHF is ok if you are further away from the boat than you feel like swimming, and a fair bit further potentially, unless it's rough. All VHFs work on a straight line of sight from antenna to antenna. If you are floating on the surface and talking on the radio, that antenna is about 8" out of the water tops. If you're in even 2' seas, the effective range is equivalent to the distance to the next wave. On the upside, the boats antenna may be 10' above the surface or it may be 50' above the surface. That helps, but if they don't find you before you drift away, you're going to be hoping for a random boat to come VERY close to you.... but, ya better than noting.
The PLB, on the other hand is kinda overkill when you're 300' from your boat and feel lazy. It's a balls to the wall, summon EVERYONE to come save your lost a$$. Knowing that, I suspect people are reluctant to pull the pin until they are really gone... Every rescue agency on that quadrant of the world is going to know where you are, but your dive boat that's a mile away won't have a clue. Some Coastgueard types will eventually broadcast the PLBs Lat/Lon and if the boat is paying attention, you'll be in good shape.
Ideally, carry both.
Or don't get lost.
Personally, when I am on a dive boat, I pass arounf Werther's toffees before and after dives to divers and crew. They love that stuff. If I'm not on the boat, someone will notice. They probably won't come back for me, but at least they'll know I'm gone!
Incidentally, I have one of those little canisters than Don linked to. It's a solid piece of kit. The guy selling them can be a bit hard to reach at times (he's a wildlife filmmaker I believe) but he's an upstanding guy. When I ordered one, it didn't every arrive. He sent another one, no questions asked.