Spanish BBs use small, custom sized bearings that cost about three or four times as much as a normal bearing; you can use a 19mm (3/4") or 22mm spindle with them.

Mid BBs use regular American Profile bearings (w/a 19mm internal diameter), which are much stronger (I've already seen a few people blow apart the Spanish bearings) and cheaper; the companies that use this BB (S&M/Fit, FBM) are currently looking into a way to use a 22mm spindle with them.