The private coupon was a total surprise to me. I signed up on Ford's web site to get a brochure and they sent me the coupon (both via e-mail and snail mail). But from what I have seen elsewhere, I was fortunate, as it appears they don't always do that.
As far as whether to buy a '13 or '14 now or later, I have been driving this awesome new ride for two days now, and I feel like a total rock star. No joke. People are calling "nice car" out to me on the street when I drive by, some guy took pictures of it in a parking lot, etc. (OK that was a little creepy, but what the heck). That's saying something since this basic body style has been around since 2010. The '10-'14 Mustang GT is a beautiful car, and compared to my old New Edge GT (a 2000), is comparatively rare - I see far more '99-'04 GTs in my locale than '10-'14s. You could certainly wait for the prices to drop on the '14s but then you may have a hard time getting exactly what you want from the leftover stock. I was very specific in that I wanted a Premium with few other options, a manual tranny and black paint. And having just turned 40 this summer (and my wife basically telling me to get one) I had a good enough excuse to take the plunge. I am glad I did.
My thought was to spring for a '13 or '14 now, regardless of how the '15 looks. I figure even if the '15 looks so awesome that I just have to have one (doubtful in my eyes if they go all "European" on me), I'd wait a few years anyhow until they have all the bugs worked out of the new model. Independent rear suspension = more complication and expense, and I am sure there will be some improvements to the platform within the first few years of sales.