I don't know, and I wonder if anyone here has heard both. What I will offer as my opinion is that for quite a while now have I always recommended the Image line over the Alpha line for a modest increase of budget. I bet when these lines were new, years ago, that the price difference was more dramatic, but now you can get Image bookshelves for $260.
There is, IMO, a very large and easily discernable difference between Alpha and Image. If you look at PSB's site regarding B1 and B15, you will notice that the sensitivity and impedance specs are all identical.
Well worth the extra $60. Remember, these are your speakers, not speaker wire. IOW, this is the place you should be splurging.
Big room, btw.