I'll be honest with you guys. I just did my blind tasting of S-189 vs. Diamond, and.... I really just cannot tell these apart. I thought maybe I could on the first sip, but after that, they really just tasted identical. If anything, I found S-189 might have had more hop flavor, while Diamond tasted less hoppy but also more bitter... but this was like splitting hairs. I really don't know if it was just my imagination. I got the same attenuation and FG with both, so I'm almost thinking maybe... these are super closely related.
Update 3 months later...
What a difference age made. I tasted the two versions side by side again. The S-189 was slightly hazy, and a bit fruity, and tasted more "like homebrew" (which I think might be some oxidation), versus the Diamond which was darker and clear, and tasted like a very lovely German lager indeed! So I've finally got my answer, Diamond is my favorite lager yeast without any doubt!
Granted, if memory serves, these were fermented warm as well, which might have added that slight fruitiness to the S-189, which I do not pick up as much in Diamond.
I've been raving about S-189 for years, and it's a great yeast. But I'll be playing with Diamond a whole lot more in future.