From a functionality and practicality perspective, 17s make the most sense. This thread has some interesting perspectives:
https://www.2addicts.com/forums/show....php?t=1582724
Here is my experience and view as expressed in this thread:
I have run both 17s and 18s on my 135i, M235i and now my M240i with both summer and winter tires. I have the equipment in my garage to change out the wheels in a matter of minutes. Here are a few thoughts:
Handling. The 17s are at least as good as the 18s even on the track. No perceptible difference.
Ride Quality. 17s are a more comfortable ride.
Noise. 17s are a little quieter.
Venerability. The 17s are less vulnerable to damage --either curbing or bends from potholes.
Tire Choices & Cost. The choices for 17s are plentiful and the cost is cheaper.
Weight. The weight of a 17" wheel + tire is usually (at least a little) < than the weight of an 18"+ combination.
Aesthetics. This is subjective. The 18s are probably the sweet spot, but I like the way 17s look better than 19s.
Square. I prefer a square set up over staggered. Probably another whole topic by itself.
Brands. I have run BMW, OZ, BBS (forged and cast), Alutec and others (forged and cast).
Forged. I have had BBS and now Signature forged wheels. Forged is the way to go if you can spend the money. Much lighter and much stronger.
Posters. Lots of great posters on this forum such as BEM-S4, Sportstick, Xutvjet, and more.
I am a function over form guy -- How it works is more important than how it looks.