Hi GNDMini
Where about's are you in West Yorkshire? I live/drive around Leeds/Bradford area and yep the roads are BAD. Have still gone for the 17's though. With the run flats having a tougher side wall it should help prevent damaging the rim's.
My Dad has 17's (45 profile) on a Merc E320. In two years he's covered 50K+ miles and the only damage to his wheels has been from his parking (the spokes stand slightly proud of the tyre wall). And I would describe his method of driving as 'not one to hang about'.
Didn't get chance to test drive a MCS with 16's, but the 17's felt fine to me.
On another note, where did you order your car from? Ordered mine from Listers (Bradford) in June, expected del for November but I still haven't got a build date yet.