Until last year, my city (Rimini - Italy) hosted the major musical instruments italian expo (DISMA) and was the only place where I had the chances to play these monsters. and you had to use headphones to try them (due to the extreme noisy place). The last one I tried was the AT-90SL and I can confirm the majority of sounds are exactly the same as the G-70 sounds, even many styles are the same. They have added the rodgers organs, some theater organ samples, and different styles, (the style editing functions are much limited compared to the G-70 however).
Other than that, the difference is that you can layer more sounds together (If I remember correctly up to 5 on the upper keyboard and 4 on the lower), and using two manuals and a pedalboard, if you are a good player you can do things that with an arranger are impossible. But with the aastronomical price they have, you could buy many keyboards and a MIDI pedalboard and do pretty the same.