When I went through this same exercise over 3 years ago, I concluded then that the ZHP was cheaper to acquire, cheaper to insure, cheaper to maintain and cheaper to operate. It's has much better ride quality, gets better fuel mileage, and yes, it's not as fast as an M3. Looks-wise, the M3 is more aggressive, but, the ZHP is still a head-turner, and I quite prefer the stock ZHP wheels to the stock M3 18's, and I prefer the cube trim over M3's solid aluminum. I wasn't looking to go cheap, but I was looking for lower maintenance, better fuel mileage, a 200K plus lifespan. Ultimately the refined ride quality is what swung it in my favor. I landed a 2006 ZHP with 40K on it then for about 2/3 of what a comparable M3 was selling for. The frosting on the cake was it was never smoked in, had no tint, was 100% stock, garage kept and dealer serviced. I never looked back, and I'll keep it forever.
Ron ///Man
'06 ZHP Coupe 6sp Sparkling Graphite
'02 330ci SE46 Racer
'03 330ci Endurance Racer