I think I'd enjoy the MX-5 as an occasional use car. It sounds like it would be fantastic. I would have no interest in driving it through an Edmonton winter though. I'd need to be in a higher income bracket to justify the cost based on the limited use.
The FR-S interested me for about 5 minutes when it first came out. Before picking that one, I'd need some kind of time machine that could send me back 30 years while simultaneously providing the income to afford one.
I haven't spent more than 10 minutes in a Turbo Mustang, but it sounds like the most well-rounded, real world choice. The FR-S and the MX-5 are hard-core sporty machines while the Mustang is definitely soft-core sporty. I think the hard core market is orders of magnitude smaller. Actually, I'd be quite surprised if many Mustang buyers even considered these other two.