First, I'll second what Lon said. Based on Steve's posts both the Sarah and the UFO25 are top of the line. Both share the Decware "house sound" and yet are different. Also, note that Sarah has roughly 2 to 4 times the Watts of the UFO25 if that matters to you.
While I can't speak to Sarah I can talk about my experience with a singled ended 300B amp similar to this:
http://www.thebestamp.com/Single-Ended_Tube_Amps/SE-300B-BAL.phpthat I bought used last year and my UFO amps that were configured as balanced monoblocs. The 300B sound is more similar to my UFOs than I expected. It is a little warmer and richer sounding, but not as transparent or as spacious. On the right recording, often solo piano or string quartet, it sounds gorgeous; the best, most beautiful sound I've heard in my listening room. Maybe anywhere ever. But overall I prefer the UFO sound. The UFOs sound lovely, and have a little extra dynamics, detail and sense of scale for the big music that I prefer like Mahler, Brucker or the Buddy Rich Big Band. I love the UFO sound so much I got back on the waiting list for a pair of UFO25s and hope to have them in the fall. Of course this is my taste, my room and all that.
I think the bottom line is as long as the 2.3 watts from the UFO25 is not an issue for you can't go wrong with either. I expect soon posters will be reviewing their Sarahs and that will be very interesting and informative.