I've had a late 2014 Cooper S for about a year/ 30000km, current mileage is 134k.
Maintenance is maintenance, and parts prices are comparable to E90 prices, factoring in the ABM discount.
The one thing I have noted is that everything takes a little bit longer to do (motor and surrounding) because of a lack of space, so something like an engine mounting on the drivers side is a 3 hour thing. I don't mind, but a shop may charge your an extra hour, as an example.
Find a good independent and a good scan tool/ or both, make sure the car is maintained as and when necessary and it shouldn't give you too much grief.
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