Hi and welcome to the forum from Northants.
I don't prefer them because they are 19's, its the style I prefer. Lots of people put CSL style wheels on Z4's, which is no problem as they suit it but the CH's just set it aside from the rest. The 230's would suit it, nice and simple and easy to clean! Not sure if the Z4 suffers from tramlining as bad as the Z3 but most of the time its down to the tyres which could have been your problem with the 19's.I agree that the 19s did look better but the tramlining was pretty bad (the tyres were good quality - Ultrac Sessantas on the front and Pilot Sport 2 on the rear, but all around 3-4mm which may have been the reason?) Also the rears were stretched slightly - perhaps to avoid rubbing - but the wheel rim protruded from the tyre and I was so nervous about kerbing them as they were almost unmarked! But if I did go for a set of 19s next, it would probably be some style 230s - quite unique but I have seen a couple of Z4s with them which looks good!
And yep it was a Royals wheel, makes a massive difference and looks so much better!