I listen to vocal jazz as well.  I have both and I would recommend the Sarah 300b.
Bob

I don't think one amp is necessarily better for a particular kind of music, it is more about the kind of sound you prefer. I think the bottom line is you can't go wrong with either but here are a few things to consider.

If you have very sensitive speakers then hum is more likely to be an issue with Sarah than a UFO25.

Do you love your preamp? I think the UFO25 benefits greatly from a good preamp. With Sarah a lot of people seem to like either a passive autoformer/transformer based pre or none at all.

If you chose Sarah you are committed to buying 300B tubes.

If you haven't already I would strongly suggest reading the Sarah and UFO25 development threads in their respective fora.

For me, I have a pair of UFO25s configured as balanced monoblocs. I wouldn't trade them for anything but of course that is just me. I haven't heard the Decware 300B amp but have lived with a different one recently. My comments are mostly based reading posts over the years, there is a lot of wisdom on this site and time spent reading will be rewarded.

I have two Decware amps, the SE84UFO25 and 300B, which I play daily at different times in different rooms.

My relationship with the Decware 300B is truly ambivalent. At times, it delivers audio that takes my breath away, offering a level of detail and clarity I've rarely experienced. Yet other aspects, namely its annoying hum, frustrate me, creating a push-and-pull dynamic that's hard to ignore
My relationship with the SE84UFO was love at first listen, and that continues.
I have talked with Steve (monthly), and he acknowledged that the 300B hums. The big question is, can you hear it from the listening position when listening to music? In my case, it depends on what tubes I'm using.
I found that different 300B tubes affect the pitch of the hum. WE 300B and Shuguang 300 B-Z hums are easier to tolerate, while the Sophia Electric tubes are not.