Got pinged to weigh in on this thread since it's almost exactly the same question I posed a little over a year ago (linked above) before going with my Duke. Just did an 800 mile Fri/Sat ride from San Rafael to Shelter Cover. It was a ton of fun and we hit some great roads - I'm looking forward to the late Sept ride that doubles the length and distance @ 4 days and ~1600 miles. That was to say, I'm fresh off of about 16 hours of seat time on my Duke and it reminded me how much I love the bike. It feels lighter than anything I've thrown a leg over outside of a dirtbike - it truly feels lighter/more nimble than my R6, 701 sumo, and R3. It feels almost on parity with my WR250X in terms of nimbleness and flickability at all speeds. The brakes are almost overkill, which is a good thing - they put my 08 R6's brakes to shame. Took me a bit to get the lever pull dialed in for a good mix of linear pull and smooth initial bite.
I wish I could speak to the MT09 or STriple directly - I've only got seat time on the MT09 (met someone with a Rekluse in theirs). With my limitations, I've had to learn to read reviews/forums very critically and extrapolate the feedback on them against my experience on various bikes to get an idea of what they're like to ride. From everything I've read, 890R or STriple RS comes down to aesthetic preference. Both bikes receive absolutely rave reviews. I think they skew slightly in favor of the Duke, generally, but that might be my bias showing. The 21 MT09 is too new for me to have a good grasp on how it ranks but it SEEMS that the refresh has closed much of the gap on the other two.... but with the benefit of Yamaha quality/reliability. Any of these bikes will be a fantastic riding experience - the ownership experience might be the decision maker for many.
But I have to reiterate the reliability issues. I'm dropping off my 890R tomorrow to have Mach 1 look at three separate oil seepage points - 2 at the valve cover and one at the base gasket. KTM's reliability has lived up to its reputation for me across 2 bikes. My 701's motor needed 2 rebuilds, a top end and 5k miles later the bottom end grenaded. I'm hoping that the 890's issues remain minor but I was weary enough to pay for an extended 5 year warranty when I bought it... would never feel the need to do that on the Yamaha and it'd be 50/50 on a Triumph.
890 vs MT09 on the FZ09 forum was, surprisingly mixed feelings
https://www.fz09.org/threads/2021-mt09-sp-vs-ktm-890-duke-r.68067/
7/890 forum had good things to say RE the STriple
https://www.790dukeforum.com/threads/790-duke-vs-street-triple.1904/#post-20310
Can't wait to see the follow-up on this post; would love to find more riders to compare notes with.
for your track bike