Considering a low F and want a solid whistle but not too loud and easy to play in the 2nd octave. Out of these two which would be recommended? Or any other ideas?
I have a Shearwater low F, I like it a lot - but I’m spoiled, as I also have an MK now, which gets most of my attention. 
Thanks! Any comments on the Optima versus Shearwater?
Well, indirectly and directly. Here are comments and performances of Optima and Shearwaters, by
a world-class performer (Orlaith McAuliffe) and a world-class whistle reviewer (Nathaniel Dowell)
An Optima Eb put through all kinds of performance tests by a musical genius:
https://www.youtube.com/watch?v=kB8h1za61Wk
A Shearwater G (and A and D) reviewed they way they should be to explain the Shearwater whistles:
https://www.youtube.com/watch?v=reEBskcJPNw&list=PLyujhpemdQEcjOVM20DGKTX2PKWz4b8Yo&index=56
Which leaves you just a note here and there off your main concern of the key of F, but close enough for
general characteristics. So onto the key of F itself:
Here’s the review of the Shearwater F, by N. Dowell: https://www.youtube.com/watch?v=b6_BQvC9RoY
Here’s the Optima F introduced and played by its maker Phil Hardy: https://www.youtube.com/watch?v=jHGwtYH2OWY