David is correct, they are totally different animals, the Audya is a Performance instrument for real time arrangement of music in live situations or for pure playing pleasure, The G6 is a powerful standalone workstation, you could produce a film score right there on the instrument but if you wanted to simply sit down and play a, it would be tricky. (unless your a Pianist)
Both can be used as a crossover and I have a friend with an Audya who uses it to produce music for Films and West End productions. So it can be done (with an external sequencer though)