If you want the car to handle as well as you can get it to handle - the choice is limited to a 15". Go with a 15x9 6UL.

If you are more focused on how it looks - then go with a 17". Just remember, the car will not ride or handle as well with a 17" as it does with 15. 17" wheels are too heavy for a Miata.

 

Don't mean to pick on the new guy but that is just not true. Grassroots motor sports did a test a few years back with a Miata adding weight to the wheels. As I recall they had 8 lbs of wheel weights on each wheel before they noticed a difference and at that it was only a 10th of a second difference in a 70 second autocross. Most guys do more damage to their time than that by the apex of the first corner! Wheel weight is important but it makes a much bigger difference in ride quality than it does in performance.

The way to consider wheel weight is in the total percentage of unsprung weight to sprung weight. Your brakes, control arms and spindles weigh far more than your wheels and few guys spend a bus load of money to lighten them.

So if you consider wheel weight as a percentage of sprung weight you might see a slight difference from a 10 lb wheel to a 20 lb wheel, probably in the feel of the steering wheel going over bumps, but a 14 to 15 pound wheel you never will notice the difference. With 400 ft pounds of torque from a V8 slinging them around I would bet you triple your current wheel weight and not tell the difference in performance.

For what it's worth I do have a little real world experience with this in my Camaro. I went from 16x8s with 245/50s to 17x9 with 275/40s to 18x10.5 with 295/30s over 2 seasons. Each move added several pounds per wheel in weight but also added significantly better performance and grip. There is no question at all that given the choice between wheel weight and width I'd go with width every time. But even so the compound of the tire makes far more difference. Race rubber on 17x9s would beat my decient performace street tires on the 18x10.5s. Yep been there found that out the hard way too...