Hi kmonroe99,
the A3000 has the advantage of tuning the scales in real time with the buttons at the left side of the keyboard.
In s775 you can also tune the scales but only when you go to menu.
The A3000 also has most style and voices for playing arabic music.
As for the functions, all Yamaha's keyboards work pretty much the same way and have similar menus.
You can have a A3000,you watch a video of how you can modify a style on a s975 or a Tyros and you understand how you can also do it on your keyboard.
So don't worry about the functions.
All styles and voices from other keyboards play in both keyboards but may need adjustments to sound better on your model.
If you like Dance music the most, you go for the s775.
If you like Arabic music you go for the A3000 to make things easier.
Both can play any kind of music.