I'm not buying or selling, I'm just getting to grips with the differences between my 80's YAS-62 and 40's Dolnet altos. After trying the Beechler mouthpiece on the Dolnet, I found I was getting a better sound than with the Selmer Soloist mpc, quite mellow but with a bit of edge so I could cut through when needed, and quite bright but not screechy when I pushed it a bit. So, old horn, slightly odd non-modern keywork (non-articulated G# key, slightly less than ideal palm key siting), sounds lovely...
As an experiment, I then moved the Beechler onto the YAS-62 and voila! A horn that's a bit better ergonomically, somewhat better-playing in general i.e. everything feels a bit lighter and tighter, and sounding almost as lovely as the Dolnet, if generally a little brighter and more modern-sounding.
So, to answer my own question, at the moment I'm favouring horn A, the Yamaha, although I could well go back to the Dolnet for an extended period, as that's what I tended to do when I was playing acoustic guitars. Sometimes I'd play nothing but the Epiphone for weeks or months, then the Crafter or Guild for a while. i always ended up going back to the Epi, though, and I suspect it's the Yamaha I will always go back to - as one or two people have said, it's probably easier to improve your sound on a more comfortable horn, and the Yam really fits my hands well, plus the Beechler mpc is giving me exactly what I want. At the moment..... 😉
Cheers, Bob