The F type is such an incredible design. I can barely think of an (affordable within reason) car that still looks so good on sale today. I'm not sure if you've had a chance to drive the manual in it but some uk reviews said in this instance it didn't add to the equation.
I am huge BMW fan boy, hence the post count, BUT for me if I was cross shopping these 2 I'd be torn.
Z4 = Tech, weight, engine and recent model
F Type = Looks, sound
Edit: just to check is it the F Type convertible? For once I think the coupe looks much nicer
Choices choices! Good times