I would go with the DX7 for the model match. Plus 72mhz will slowly fade away and be hard to find as demand falls off. 2.4 prices will go down as demand increases just like HD TV's, brushless motors, ESC's and the like. It isn't going to happen all that fast, but you might as well plan on the future and 2.4 is going to be the most used eventually. I also have a Dx7 and love it. I've had no problems with it so far.

Just my 2 cents