Yep, I agree with the 321 vs 641 distinction. The nimble, small-bore 621 and 321 have the same personality but slight cosmetic differences, and they're both very different from the 641/642 in physical, playing experience, and sound characteristics.
Someone expecting a 621 to voice like a 641/642 is going to be very disappointed.
But I had a 2-decade, no-time, post-college period, and I could pick up a 321 for a gig with very little preparation. My 967 was in storage because it required too much face-time for me to stay competent on it.
The bore difference between the 321 & 621 is in the 4th valve loop - .661 vs .610, I think - and maybe it was a placebo effect, but it felt to me like the 4th valve spoke better/easier on the 621 than the 321.
Also, the 621's default large-shank receiver makes a big difference too, but the 321 can be re-fitted to the same effect (I compared my large-shank 321 head-to-head with a small-shank 321; significant, worthwhile upgrade, imo).
In short: I think one can get most of the 621's benefit at a lower cost by buying a used 321 and installing a large-shank receiver on it.