Hey toobad, thanks for the help. I must admit I am struggling a bit with the final paragraph, do you mean it will be a really hard ride, if so I might back down to the 20" or the 19" because as well as a bit of a blast for me, I need to make it an ultra comfortable car for the family on long trips. Any advise welcome!
OK, my last sentence was written with my tongue firmly in my cheek but at the risk of starting a flame war, I will offer my thoughts on the subject. Large diameter rims and very low profile tyres have migrated from the world of race cars where they provide excellent steering response and feel, less rolling resistance, better cornering stability and sufficient clearance to fit much larger brake calipers. If you always drive like you are on a racetrack or are a salesman in a BMW then this combo is a no-brainer for improved functionality. However the popularity of large rims on the street is driven more by a styling fad and very clever marketing to sell you a dream. The cost to a manufacturer of increasing a wheel rim or tyre diameter by an inch is unlikely to be more than Ł10 and that is the pricing differential Mercedes is buying them in at. Now look at the Mercedes pricing for upgrades and you can immediately see the opportunity for high margins.
I am a hopeless slave to form over function and do like my vehicles to stand out from the crowd. I was quite heavily into the custom Harley-Davidson scene and when I lived in London I would arrive at the Hardrock or Ace Cafe in a crippled state such was the ridiculous lengths I would go to to change the appearance of a bike to something I considered more aesthetically pleasing. I always said, it's not the journey, it's the arrival that counts
Even now, with Red Ken, my poverty grade 220d Sport, I had to paint his brake calipers red and de-badge his rear end. When the GLC63 launches, how much of a try-hard am I going to look?
Wearing my sensible head, I would definitely steer you to 19" rims. I think your 43 in Sport+ mode will provide you with sufficient control way beyond your abilities in any road-going situation. Your family will thank you for it when they are making long journeys with you and, of course, there will be less propensity to skip/clonk due to a large tyre's sidewall ability to better absorb lateral forces. Spend the money saved from your rim upgrade on a professional front/rear dashcam installation that has the potential to save you a lot of money and heartache sometime in the future. Just my Ł0.02 and now ducking for cover.