When I set out to buy a C soprano, Bruce Bailey told me that he had seen intonation issues on Conn and Buescher Cs, and that Holtons were the best - plus they (at least some) are keyed to high F. So I went Holton - I did find the low Bb was reliably sharp (fixed with a liner) and the rest of the horn is very well intonated.
I haven't ever played a Conn or Buescher C, so have zero personal information.
I really like the sweet delicate tone of a C soprano, even at my poor level of playing.
Just a couple of points I'd like to clarify:
1. Buescher did make high F C soprano. Not a lot, but some.
2. Re: intonation. Any horn can be out of tune. Any horn. And sometimes it's not the horn itself, but rather what has been done to it.
My Conn C soprano was unusable when it was first overhauled. Why? Improper key heights. Had I not played one that was properly set up, I could have easily given up and said: "Some Conn C sopranos are horribly out of tune. The one I bought was."
Instead, I got the heights from the one that was properly set up, and my tech set mine up the same way. The result? Mine plays perfectly in tune. (When I have practiced properly and have the chops for it.)