I have to slightly disagree here.
I do agree that of the 2 cars the RX-7 has more value. But by getting rid of the Stang that makes the RX-7 your DD thus negating all the reasons for keeping it. So if you ditch the Stang I would suggest getting some cheap reliable transportation as a DD because there is no way you should be driving this car everyday and racking up miles.
The plan is to DD it until my wife gets a new car. That should be by the end of the year hopefully. That way I can use her old one as my DD.