I think they both have their uses/purposes. Bb Contra is nice because you can give them the bass clarinet part (since there is limited actual contra music much of the time) and they'll play an octave lower and a a nice resonance. It's a lower clarinet than the Eb, so it is 'bassier.'

The Eb contra is also going to add bottom, but can read bari sax parts or transpose tuba parts into treble clef. That can have its own advantages there to make it easier to cover missing tuba skills.

We have 2 old contra-altos, but no Bbs at my school. I think if I was going to purchase I would do Bbs myself. They're so pricey though that I don't see us making that purchase anytime soon for our program!