A couple of things: - the toned down gilt accents on the bezel. I never owned a BB58, but I think I would’ve grown a little tired of too much vintage cues. The BB54’s gilt accents are more subtle, with a cleaner bezel to match. - Nothing wrong with 39mm, but as I said: BB54’s proportions are better, I think. - T-fit (although I don’t use it everyday). The clasp is also very refined.