yeah, there are a lot of good opinions in the stickies. But here's something I don't remember reading (forgive me if it was mentioned, I don't have time to go reading through all of them again):
It's hard to empirically say what's better, low A or Bb, because in many cases the horns themselves are so different. My experiences are with Yamahas (YBS 62 and 52), school-owned, and when I made bari my main axe and bought my own pro horn, I got a very early Conn 12M. To my ears and those of my peers, the sound of the Conn just engulfs the Yamaha. Is that because it's lowBb or low A, or because it's Conn vs. Yamaha? don't get me wrong, YBS 62s (and 52s) are great horns, but for me, the Conn is the way to go.
it seems to me that the way to really test lowBb vs. A would be to try two of the same make of horn--Selmer Mark VI or P Mauriat or something like that--one low A and one low Bb. Then you could limit the variables when deciding which one played better.
I'll tell you what has worked for me. once I decided that I must have the Conn sound, I decided to work around the low-A issue. I play bari in a strong college band and gig around town on a regular basis, and to me, the sound of the Conn on 99% of the notes makes up for lack of low A on 1%. You can get away with making a low-A extension, which I'm sure they talk about in those stickies. I have one and it works great. The only time it doesn't work is when you have low A and low Bb in the same passage. If that happens, and you can't use the extension, you can either play it up an octave, play the note a 5th above, or stick your foot in the bell and drop your embouchure to get the low A (an idea I got from Barry Sachs, I think in those stickies, and that guy knows what he's talking about),
in all the big band charts I have played on bari, only one time have I encountered a spot where both the low-A and low Bb were both crucial. it was a Phil Woods chart that had an 8 bar soli where the bari and bass bone quote the Bird intro to All The Things You Are. well, the first phrase in that quote ends on Bb and the second phrase ends on A. Clearly Phil wrote for those two instruments because he wanted the low end to come out, so it wouldn't make sense to take it up an octave, and it was too fast to put in the extension. your "foot in bell" chops would have to be pretty good to play that line on a low Bb horn. luckily I had the Yamaha for that gig. but other than that, I think it makes more sense to have the horn that you prefer all the time rather than the horn that you don't prefer but can get that occasional note (especially since the low-A extension allows you to to play that note in 95% of places where it's needed).
In rock/Motown stuff the low A is absolutely crucial, but again, the extension works great in those situations...since that music is often diatonic, there are only two keys that on bari would need both low Bb and low A--concert Db and concert Ab, not the most common keys in rock music. So usually when you absolutely need the A, you won't have any Bbs so it will work fine.