As with most of my posts I am writing in glorious ignorance of the actual differences between the 4 pot and the 6 pot engines. My Zed is my second BMW - the other being a (6 pot) 328i Coupe. I remember thinking that I'd never be able to afford to insure and maintain the 328ci, yet it was one of the less expensive cars I've had.
I did once have a 1.9 litre Z3 as a courtesy car - that was gutless.
Actually, speaking from a sample of one, the Z4M hasn't cost a lot to run and now that I'm an old bugger with a clean licence the insurance is only around £220 per year. But the Z4Ms, as previously posted, do take you into another place performance wise.
The BMW 6 pots are great engines and with sensible maintenance they seem to go on forever.