I have heard both flying in Australia...
... In Australia group form is the preferred form of transmitting callsigns (paraphrased from AU AIP GEN 3.4-20 para 4.16.1). As examples are listed Sunstate 122 as "Sunstate One Twenty Two", and Car 21 as "Car Twenty One".
There are no examples in the AIP of callsigns involving 9, however I can recall hearing all of Thai Niner Niner Niner, Thai Nine Nine Nine, and Thai Nine Ninety Nine when on frequency.
Maybe an Australian ATC can elaborate what is more correct? I guess the main thing is that the message gets across...