Wow, this line alone makes this different than all other UGS threads!!!

I would say, estimate your total spend for the year excluding this ticket. If it looks like it could use a nudge, get the Z fare. If the marginal spend on this fare is not very material, get the B fare and save yourself some cash.

I agree with wimpy that it will probably not come down to this ticket purchase

My $.02 from a non UGS.