Without a doubt, I'd go with the Roland BK-9. I wouldn't want to do all backing tracks and give up playing styles. That would be to restrictive for me. I have nothing against backing tracks and actually use a few here and there, but I would always want the option to play with styles.
As both methods of play have their place and restrictions styles are very repetitious but flexible, & backing tracks are strict to arrangement perimeters & you can use many more tracks and parts to make it yours especially when singing.....
Mix it up