I believe having 255's at the back aids traction (but worse in snow) and also improves low speed comfort as they will run over small road imperfections whereas the 225 is more likely to go into the hole.
It's the simple logic of more rubber = more grip and with being more than happy with the staggered setup on my 330d (which leaves me in an hour or so
), I made sure I had it again on my 328i.
Cars with skinny tyres on the motorway aren't much fun (not that the 225's are skinny) so using that train of thought, I want some fat 18" out back. Which here in the UK, I can have!
I believe the 19" will compromise comfort too much.