X90 is the competing service run by the Oxford Bus Company (Go-Ahead), whilst the Oxford Tube is Stagecoach's offering.
The Oxford Tube is all double decker, the X90 has recently-ish started running some services with double decker coaches which are shared with their Oxford to Heathrow/Gatwick "airline" services.
Yes Plaxton Elite interdeck. Not really a double decker as there are no seats on the lower half unlike on the Astromegas.
Sam