Aaron,
I’ve gotten out of the business of writing reviews and giving advice on the board, however, since very few people on the board have played the Grinter, I’ll make an exception. I also have Phil O.'s old Chieftain F, so I’ll comment on that as well.
Phil’s Chieftain F is indeed unusually good for a Chieftain (they tend to be very inconsistent), however it’s no match for the Grinter or Overton Low F’s, so let’s get that out right away. First off the Chieftain’s holes are not drilled perfectly in line (they wander off to the left), and second, while the Chieftain plays well and sounds good, the Overton and Grinter have a much more complex (and to my ear) more pleasing tone, there’s more depth to them.
So then, between the Overton and Grinter, it really boils down to what you want: Volume, backpressure, the ability to play for a long time on a single breath, durability, low maintanance, agressive “growly” sound and moderate price? If that’s your bag, go for the Overton.
OTH, if you want beautiful wood, zero clogging, amazingly sweet tone, don’t mind lots of oiling and worrying about cracks, and have a big fat bank account, then the Grinter may be for you.
Both are really, really awesome instruments. Personally I found the Grinter Low F to be a bit of an air hog, much like the Shaws, and the one I had didn’t like playing the last three notes in the second octave - an observation echoed by another Grinter owner I spoke with - not Phil though. Still, the tone of the Grinter whistles is freaking amazing - it’s one of the only whistles I’ve ever regretted selling, and I only did so because I needed the money at the time.
Having said that, if I still owned the Grinter, it would likely see a little less playing time than my Overton Low F, the Overton is simply easier to play and also sounds great, but in a different sort of way.
So there you have it: You need both 
Loren “Grinterless” B.