I think there are quite a few things you need to consider. What attracts you to the mustang? Is it the engine options? Is it purely the styling? Do you care about the solid rear axle of the current cars? Do you have really specific requirements about what you want in your dream mustang? Do you care about whether you buy a new or used car?
Styling vs Engine: To me, this is where you consider whether you want a muscle car or a pretty car. I don't think the new mustang, at least when initially released in the US is going to offer any crazy engine options or remove the V8 in the GT. There might be a ecoboost 4 cylinder on offer, but that seems up in the air for the US market--at least from what I've read. But if you really dig what will more likely be a more retro design in the current car, that might be the way to go. If you're really interested in engine tech more than appearances, it might be better to wait until Ford at least announces what will be available in the '15 cars
Solid rear axle: I own a '12 GT. It's a great car. But the rear axle is by far my least favorite aspect of the ownership experience. I didn't pull the trigger already and was in your shoes, I'd probably wait to see if the IRS is that much better than the SRA.
Specific requirements: Are you a guy who buys off the lot or are you someone who wants very specific things and doesn't like to compromise. If you're specific and you wiat too long, you won't be able to order a '14 mustang and will have to deal with what's on the lot. You might be better off buying now and getting exactly what you want. I'm not that picky to be honest--I don't care about color (that much) and I don't like a lot of convenience items. And yet, I couldn't find a base model GT+ brembo package+3.55s when I got my car, so I had to order it. All the cars in the 3 state radius are either autos, sticks with all the convenience items and no performance options, or fully loaded with everything. I didn't like my choices.
New or used: If you wait to see what the '15 will be like to actually drive, there may not be any '14 mustangs around to buy. You could always buy a used 13/14 mustang, however. But some people don't do used cars.