I really suggest waiting for the 2010 mustang because it looks much better. Just hold out for a few more months.
But that is a hard question. It really depends on what you want to do to the car. If you are planing on modifying it to make it fast, I suggest getting the coupe. The coupe is much more aerodynamic and weighs less. If your just looking for a car to cruise around in or for commuting, then go for the convertible. There's nothing like cruising around with the top down during the summer.