We just went through this same thing a couple of months ago. My wife wanted a convertible sports car and those were the primary two that came to mind. A friend owns the Pontiac so we had that for comparison. We drove several Mazda's and a couple of Pontiac's. Our conclusion was the Pontiac was more fun to drive but, with the fold back top, zero trunk space. Power was decent but not overly impressive. Handling was good for what it is. The Mazda was a blast to drive. Plenty of space inside, handles like a go cart and power was maybe a little less than the Pontiac. I still liked the Pontiac better. Wife decided she didn't want a car just like her friends.....sheesh...

So, we decided to go with the Mazda but had a very difficult time finding one that wasn't ragged out. Way too many younger people buying them and playing Fast and Furious I guess. In the end we bought a Chrysler Crossfire. Yeah, I know, it wasn't on our radar either but let me tell you it is one hell of a good car. Blows away either of the other two hands down, every category. That being said, if I had to choose between the original two options, I would look for a very clean Mazda and definitely check the Car Fax on it.
Good luck and happy motoring!