I had G70, E80, E50 and BK7M. There is a small difference in sound between E50 and the others, but only to people like us. The difference between the others is most likely the way they are set up or tweaked.
I have recordings of myself on all four units and can't tell the difference listening to them, except some of the lead sounds on the BK seem better. I also think some of the factory settings are better on the BK, as well they should since it's the newest of the lot and they have had more time to develop and perfect them. I'm talking about the way the styles and sounds are set up from the factory.
To be fair, I've experienced exactly the same thing between generations of Yamahas and Ketrons. The later models have more evolved sounds and effects that are triggered via pressure, switches, buttons etc. Other than that it's truly hard to tell on which model the songs were recorded on, with the exception of the Audya (live drums).
They are all wonderful.
I prefer drums from Ketron, Roland, Korg, in that order, but all are satisfactory. Others will surely have a different order of preference and include Yamaha.
I'm now after optimum size, weight, and ease of operation. The PA3X is winning that war, taking into consideration that it means I don't have to carry a laptop for text files and MP3 playback, an external mixer, an external screen, or vocal processor. It has a very good lightweight pedal, (EC5), and facility for 2 more foot controllers if needed. It has the joystick I prefer that is located where I want it, it has great factory support and a good record of reliability. It has multipads. It is good breaks and fills, intros and endings. And Korg has a history of providing free updates and upgrades, as needed. It is easily edited. Far from perfect, but the leader at this time for my needs.
I wish I had an Audya 5 too though.
DonM