Came across this thread a year late, but I'm sure it could help someone else out in the here and now.
A work contact of mine found out I was a hunter, and brought up the Swaro BTX he owns. He insisted that I borrow it for the season, and how could I say no?! He also lent me the STX eyepiece with his 95 mm main body. Last night, I decided to give it a look, reaching out about 10.4 miles, into an area where I glassed up a very nice buck only a couple days before. Mind you, I was just over 1 mile away at that time, but I thought it would still be a fun exercise. I only looked through the BTX on this particular evening, but decided to break out the scopes again this afternoon for a few moments. This time, looking through the STX 95 and 66A, and only for a short period of time.
I should also note that I've owned the Kowa 66A for a bit over a year, and have put a fair amount of time behind the scope looking for mule deer. I've really enjoyed the scope, and don't really have any complaints to date. Perhaps that will change as I put more time behind the Swaro, but only time will tell.
First thing I noticed between the two was the zoom mechanism. With my copy of the 66A, I've always struggled to get that scalpel sharp focus everyone talks about. Don't get me wrong, the "glass" is fantastic, but I struggle getting it perfect. Every once in a while I nail it, and boy, is there a difference! Something about the fine focus wheel just doesn't jive with me, despite my original thoughts that I'd like it way better than the more common barrel style. The focus wheel on the STX 95 is much easier for me to fine tune. And unlike many people out there, I get more shake when focusing with the Kowa than I do with the Swaro. And thats even with the Swaro mounted to an UL Sirui T-024SK and a cheap Small Rig pan head, while the Kowa is on a Tricer AD with the LP pan head. Its not the tripod either. Each time the Swaro is on the Tricer tripod, I find it even easier to focus without a ton of shake. Perhaps that just personal preference, or may even a focusing technique thing on my end.
I know this wasn't part of the OPs original inquiry, but zoom equivalent was quite interesting as well. I will admit upfront that my "test" isn't apples to apples, but the copy of the STX that I am in possession of seemed to be VERY different than my Kowa. I had the STX 95 backed down to 50X, and it still seemed to still have a slightly larger zoom than the Kowa 66A at 60X. Perhaps this is due to the difference in objective lens size, but I am far from an optics expert, and can only speak about my observations, not why they are so. I didn't spend a ton of time analyzing the associated field of views, but a quick seemed to show they were nearly identical at the 50X and 60X as mentioned.
The last thing I came across was cutting through mirage. Given the short observation duration this afternoon, it likely could have been the focusing issue that I covered above, but after observing a fallen tree at the 10.4 mile mark, the Swaro seemed to be a touch sharper than the Kowa. Again, they are very different objective lens sizes, but the log was just a tad sharper through the STX. After a tiny bit of fiddling with the Kowa, I did narrow the "gap" by about half, but the Swaro still came out on top. Perhaps I got one of Kowa's lesser copies, or even one of Swaro's superb copies, but there definately seemed to be a difference in my eyes.
I plan to continue my evaluation when I have more time to due a comparison justice, and under better glassing conditions, but figured my findings here might help someone in the meantime. Will you be using them at 10.4 miles? Probably not, but maybe. And I'll also mention that I was hoping the Kowa would come out on top. I could even go as far as saying I was a tiny bit disappointed that the Swaro edged out my 66A, but maybe thats just me hoping my 500 HP Nissan could edge out the 700 HP Mercedes AMG; not a perfect analogy, but you get the idea.