I've only driven a Miata a couple of times, but my impression is that it's much more cramped inside compared to the Z4. The materials also don't feel nearly as high grade as in the Z. But one thing to consider is the maintenance cost of the Z4 once it's out of the warranty. My suggestion is to get the extended warranty (for a maximum of 3 additional years) before the stock warranty expires. Shop around different dealerships, you can find the Gold (if you don't have iDrive) for around $3000, and the Platinum (covers iDrive and other electronics) for a few hundred more.
2009 Z4 sDrive35i DCT, AW/CR
2010 335i Coupe 6MT, SG/CR [SOLD]