If you want a go 19, just powdercoat the stock rims. In that way you will save more money and stay with stock tires. But if you plan to purchase the wheels, definitely go with 20.
Waiting New M4
Sold- 2011 535 Xdrive Msport TiAg
Retired- 2008 535i Sport Jet Black
Retired- 2006 335i Sport Jet Black