Original DX7 is an older 72mhz JR radio with a spektrum module added to it. uses JR programming which in my own opinion is more difficult, and doesn't balance right because it is a conversion of a 72mhz radio to 2.4ghz. But it's a great high quality radio.
The DX7s is Spektrums in house designed 7 channel radio. It uses Spektrums airwave programming system which is really easy to understand, balances properly from the neck strap, has two way telemetry functionality, and can be software upgraded at home using the built in SD card slot. Spektrum is still suffering some growing pains when it comes to quality control, but If you have an issue they WILL take care of you.
If it was me, I'd go for DX7s