I like them both and buy whatever is on sale. Love to eat them both. BB have more of a milder pork flavor but the spares taste more like ham. I always trim spares down to St Louis cut but then cook the end pieces right along with it. Seems like extra work, but they seem to cook up more evenly that way.
I used to cook a lot more BBs than spares because they were easier for me to cook them right. Once I quit using a rib rack, all the problems with getting spares right went away.
WSM, 99 & 09 Performers, 2-OTG, SJG mini WSM, Summit 420, Q100, Akorn, and Schwenkgrill