Although I think that you will get some pretty biased (but true answers) on the fourm, I think that you have all but answered your own question. Z vs. Challenger. Well, I am not a huge fan of American cars, however, besides that, have you seen the reviews that the Challenger has gotten in various automotive magazines? The last article that I read was from Car and Driver, and it didn't do so well. They basically likened the thing to a tank, and it has not produced any type of stunning numbers.
If you are worried about fuel economy, you shouldn't be looking at any one of the two cars that you are. Both were not made to get the most MPG, however, I do not know, but if I had to guess, the Z would out do the Dodge.
I haven't had any problems with the Z as a daily driver. I work as a firefighter in the Metro DC area, and have to haul around firefighting gear, uniforms, meals, sleeping stuff, etc..., every day that I work. Amazingly, the back cargo area actually holds my gear, I am still shocked. You already drive a 2 seater car, so I don't really see how it will be such a huge shock for you.
If I were you, though, and I could pick, I would choose the Z. I really feel it is a better all around car. However, what I would do if I were you is go to both the Dodge and Nissan dealerships, and see which one cuts you a better deal! Good luck!