Todd, I've read that those whistlers can get very loud, and whistle the whole time you're fueling, changing pitch as the tank fills up. Other than that I am pretty ignorant of them. I just got a new boat mechanical book in today, and it details the Racor LG100, which looks perfect to me, other than being quite large. Racor makes an LG50 for gas only that looks much more reasonably sized. I think I can fit the LG50 right into my existing vent lines. They're a little spendy, but with their ball check valve that will not allow fuel out when filling, it looks like exactly what we're all looking for.
BTW, the new book is "Boat Mechanical Systems Handbook" by Dave Gerr. It's not nearly as useful as Calder's book for us, but I've only glanced at a few select sections and have already come up with some useful ideas, which as you know, are hard to come by.
Another good book I just got in, recently updated in 2019, is "Happy Hooking - the Art of Anchoring" by Alex and Daria Blackwell. Excellent information about everything to do with anchor systems. Lots of useful information here.