I would start by saying get a more reputable brand of wheel like HRE, Volk, Advan, BBS... Secondly I would say that it's true that both of those sizes would work but it's hard to give sizing advice without the wheel width and offset.
'08 E90 M3 - Alpine White - Sold
'13 E92 M3 - Silverstone II - Sold
'13 E92 M3 - Alpine White - BBS E88, JRZ, Akrapovic, AP Racing
'04 E46 M3 - Jet Black - BBS E88, KW Clubsports, Brembo BBK, SuperSprint Exhaust, Recaro - FOR SALE