In my personal experience Tifas ultimate weapon does more damage the closer to limit break you are. That's why I dont use her.
Anyway, if using Vincent alot in the game and letting him kill alot, he gets really powerful at the end of the game (thing is, you got to inflict sadness on him cause really, you shouldn't want him to get a limit break). Yuffie however, is powerful, end of story.
I dont like any of their limits.
Vincent and to the plot and is really cool, Yuffie is annoying.
What they are great for though, is morphing stuff.