RE: AR9000 versus AR8000
The AR8000 is a newer Rx than the AR9000. The AR9000 is DSM2 and has been around for at least 4 years. The AR8000 is a DSMX Rx and was just introduced with the Spektrum DX8, this year. I do not have a DX8 or an AR8000 so I cannot speak to the differences. Both will work with a DSM2 transmitter.
Perhaps Andy Kunz will pop in and address your question.