My experience has been that SBA altos are much better horns than BA. Note this applies only to the altos. Better build quality, noticeably better tuning, and I prefer the tone more on the SBA. Plus, while a BA alto with high F# is -extremely- rare, the SBA altos are available w/F#. The Selmer catalogs mention some tuning improvements in the very late BA altos, so I guess even Selmer thought there was some improvement needed on the altos.
All of that said, for a vintage alto tone/approach, my preference is an SBA neck on a modern Series II alto. Best of both worlds; modern tuning/ergos, vintage sound.
Both the BA and SBA altos are quite sharp above C on top of the staff. This is better in the modern horns(still not what it should be, tho), so at least Selmer's making a bit of progress. And no, the sharpness is not my mouthpiece or manner of playing. I've been around long enough and can play well enough, and have tried other horns(with better tuning) to know what I'm talking about here.