I know what you mean. The problem with 18's is that the speedo will have to be altered and probably the wheel arches. That will be an expensive job, something really you don't want to mess with, especially on a new car like these beautiful minis.
I'd go for the 17's and maybe lower the car 20-30mm as well, but check with a pro first. That means hopefully you won't have to alter anything major. I had to on my Golf, it was a pain I could have done without. 17's will look great trust me.
Good luck!
