You might try m3forum.com, it has been discussed ad naseum there, just search. Basic idea 18s for performance/available rubber, 19s for looks, ride is another factor but most don't find that much difference, either way it is pretty taunt but I appreciate it. I personally have OEM 19s for street and light 18s for the track.
I have aftermarket 18s, the OEM 18s are cast (19s are forged) so they aren't much lighter than the OEM 19s. Since I don't have the OEM 18s I can't comment on the ease of cleaning BUT I will say a lot of people have been experiencing chipping on the OEM 18s finish.