I would stick with the BC26II, especially if it sounds incredible.
Go with the known.
I have been watching for a used BC26 on Audiogon for the last 10 months. They NEVER come up. That has to tell you something.
The BC28 comes up every couple months or so.
I bought one. Sold it 3 months later.
Not a bad sounding amp overall. But finding the "perfect" 6922 tube drove me nuts.
Always a compromise somewhere - bass, mids, body, soundstaging, imaging, etc. NOS 6922 are getting crazy expensive as well - $300 to $400 per pair. I am done with tubes (for now)
From all accounts the BC26 has "tube like" body and imaging.
I would love to audition one. I think it has a bunch more power than the BC28 as well.