I am buying an 06 MCS cabrio with JCW kit and JCW 18 inch wheels. I have only driven previous MINIs with 16 and 17 inch wheels with and without runflats. This car has runflats. What are the advantages of 18 inch wheels?

 

The main advantage is that they cost more.
Oh, you mean the advantage to you, the buyer?

 

Looks... if like that look, and ummmm, more momentum maybe?

 

18" wheels are popular because:
They are not easily found on middle to lower end cars.
Big diameter rims are in vogue- they are actually oversized
Larger wheels look different and "impressive".
They are easy to spot as being not normal (stock wheels)
Larger wheels show off the brakes and give more room for larger brakes
You can usually run really low sidewall tires like 30 or 35 series- stiff sidewall gives quicker turn in but more harshness to ride quality.
You can save the JCW 18" wheels for car shows.

For your particular set up it will be no worse than any MINI with stock suspension and 17" wheels with runflats. Just don't drive too fast over a pothole or bump.

If you do change to non runflat tires 215/35-18 or similar you'd probably do pretty well. If you want the smoothest ride for street use then 16" and 205/55-16 non runflat All Season tires would be worth a try.

18" wheels have some drawbacks-
Unless they are forged alloy they can be very heavy.
Forged 18" or larger wheels tend to be very expensive
18" wheels are often found in wider widths- for the wide tire look
For the MINI you cannot easily use tires that have more sidewall like 45 series to 60 series. There is not enough room in the wheel well for clearance.
Larger wheels allow a clear view of your brakes- now you have to decide on upgrading them to a big brake kit or not.
Larger and heavier wheels take more energy to accelerate- now add more power and more effort to stop- now add more braking power (BBK or better pads/fluid/etc)
Handling might be good due to lower tire sidewall but ride quality can be fairly rough depending on your suspension- lowered suspension and 18" wheels will be much rougher than stock.
Tire selection for 18" wheels is not as good (it's improving though) and more geared to performance, be prepared to have to buy tires more often.
18" wheels are not suited for snow. They do ok in the rain.

 

18's are way better looking for one. They also have vastly improved steering response and resolution due to the reduction of tire sidewall flex.

--->hune: I used 18"s all-seasons in the winter. They worked almost as well as dedicated 17" snows while looking awesome doing it.

 

you can use larger rotors... other then that, appearance...if that's the
direction you're heading.

im not really sold on the snappier steering response. the MCS is so
nimble i reduced the feel using GS-D3's 205/50/16 size. much
better stability, imho.

 

These are the factory JCW wheels over factory JCW brakes. Does that change anything?

 

not really. so you got those heavy JCW wheels and JCW brakes.

i think my stock brakes under my 16" SSR Comps brake better. (not sure,
but just a feeling... no data).