Back when I decided to get into some kilt related formal stuff I looked at the Sheriffmuir and Regulation doublets quite a bit. Based on everything I've seen on-line and in person, and read about, they seem to be used for very similar 'levels' of formality. I've most often seen the Sheriffmuir worn with 5 button waistcoats in black or tartan, and the Jabot. So the Sheriffmuir seems to generally be considered 'white-tie' like the Montrose.
I've most often seen the Regulation worn with black-tie and 3 button vest basically like a PC. I recall one vendor calling the Regulation a Sheriffmuir with PC lapels. So it's level seems to be viewed more as 'black-tie'.

I have seen the Regulation worn with a white vest and tie to 'elevate' it, and the Sheriffmuir worn with a wing collar shirt and rouche tie, I guess to lower it's level of formality . Based on that, I guess the Regulation, like a PC, is more versatile, but truth be told, I think the look of Sheriffmuir more.

In the end I didn't go with either one though...