Mustang all day everyday. The Miata is a lot lighter which is great , but it also also feels cheap to me. They are a cute car for auto cross,track days though.
I have this exact decision: I currently have a 2004 (nb) Mazdaspeed MX-5. I autocross it, however I don't track run it as I cannot pass the "broomstick" test ... Helmet won't stay under a broom stick set on the windshield to the roll bar. I am just too tall for it.
No issue for Autocross, but no bueno for track days.
Thus the MX-5 is on the market and a EB/PP Mustang is on order.