Got a Z3M Coupe last year. 14 years old & 49k.
In that process I almost talked myself into a Z4M. But the Z3M was the car I always wanted.
In terms of price they are very similar & if you want an S54 engine in a Z3M you are looking at much more than a Z4M. I think the Z3M Coupe has already leveled off in the price stakes where as Z4 has yet to level off.