It depends on the engine and transmission in each car. You can compare the two cars here:
http://www.fueleconomy.gov/feg/sbs.htm
This picture of the Miata seems to show the top stowing behind the cockpit, not in the trunk.
http://en.wikipedia.org/wiki/File:Eunos.jpg
The extra fun of driving a Miata would be worth a couple less miles per gallon. It's 5 years newer, too.
I also can't see how the Mazda would be harder to fix, since the engine is installed the correct way, instead of sideways. And the Camry is 100 times more reliable? I doubt it.