Now I’m looking for a Low F whistle. I’ve heard some good clips of the MK Pro and the Kelpie. I would prefer the lower cost route; however, if there is a demonstrable difference between the two, I would opt for the MK Pro.
These days my primary audience is my wife and dog. Any suggestions? Thanks. 
Dump the wife and keep the dog.
(That was too easy.) 
Good one! 
The main difference is that the Pro is tunable and the Kelpie isn’t. Tone and voicing are pretty standard on Misha’s whistles - gravelly is the word I use to describe them. Great for jazz. All of the MK’s I’ve played play well, with a couple seeming to stand out a little above the others. I own a Kelpie in F. It’s our great dane’s favorite of all of my whistles, and she loves to croon along with it as best as she can manage. It’s the one whistle that seems to inspire her in a way that none of the others do. My Great Dame (the wife) loves it too - and it’s much easier to play for her since she prefers either the guitar or a whistle of her own when attempting to accompany me.
Buy the Kelpie - it’s all that you need in a whistle.
And keep the wife. You may find, as I do, that the only time you can enjoy playing the Kelpie around the house is when the wife is out walking the dog…
Yep, I’ll keep her.
The main reason I gave up whistle was my two German Shepherd Dogs. I had a disability that kept me house-bound. When I played soprano D, I was treated to enthusiastic full-blown howling. My MK Pro D brought out their soulful side; the male sang in a mellow baritone and the female, a whiskey tenor; a rather good duet.
This was amusing at first but soon became tiresome; being sideman to the dogs. As fate would have it, the female went to dog heaven a few months ago. So I began to get back to the whistle.
I picked up a Goldie tenor D from Doc Jones and now I can get a fine vocal from the male and I can even get him to stop. With this new cooperation I thought I would have a go with an Alto F. A soprano D would be pushing my luck.
I do like a complex timbre; is that what you mean by gravelly?
I think I’ll get the Kelpie. Thanks for your insight.