I know this doesn't help much, but I've actually looking to do this same move since I received my car a couple of weeks ago (with stock 17's) and have really been torn between going 18 or 19.
18's just don't have many options available, but are the more affordable of the 2.
19's definately seem have many more available options and look better, but I have read many complaints about the lack of ride quality and I'm not sure how comfortable I am with the lack of sidewall protection to the wheel.
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2020 Z4 M40i - Alpine White
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