What the.. you really can't cross shop these.. they are completely different cars.
Z4: two seater, turbo
M3: four seater, NA
Forget the specs, the feel of the two cars is going to be drastically different.. nevermind the powerplant, the geometry is completely different.
Since the convertible is useless to you, IMO it would be more of an annoyance than anything. Also ignore the fuel economy thing, that's not a reason to cross shop two sports cars. I'd go M3 personally based on you not liking verts.. dunno tho.