Nice rig (I'm assuming that your guitarist isn't going through an Ashdown setup). What head are you running?
I've got a 210 right now and trying to decide whether to add another 210 or a 115. I hear that the two 210s work together to make B-string magic, but I like the idea of that bigger cone as well...
Even though you're not really asking me, my advise would be to get a second 210, and keep speaker size the same.
Unless you are going to bi-amp and split the signal somewhere along the line, the idea of a bigger cone is all marketing magic, and mixing speaker sizes is generally more trouble than it's worth.
Even if you were to bi-amp, you should remember that most bass cabinets are designed to handle the full range of frequencies that you instrument will throw at it.
IMO etc
Hope that helps
