If you included the YS 110 and the Saito 100 because you're considering them, one of them would be the logical choice since the day will come when you'll want a more powerful engine. Performance wise, there is more difference in performance between the Saito 100 and the .91 than the .09 difference in displacement would indicate. Powerwise the YS 110 is the hands down winner.

PS, as to the OS .91, I would pick the simpler of the two.