I think Ro's edginess was more due to the character being younger and angier than Kira, Ro felt at odds with most of the TNG crew and was very impulsive with how she interacted with them. Kira was similar in the earlier series of DS9 but grew into a more mature character who was more level headed, a good example is Rocks and Shoals, with Kira having to decide how to deal with the Dominion Occupation.
Honestly, i think Michelle Forbes is a better actress and more formidable than Nana Visitor.
I agree with Devileyes here, both were great in their respective roles and both could be very formidable and strong when the episode allowed. One disadvantage Forbes had though was that she got a lot fewer episodes where her character could be truly strong and passionate, not having a fully developed Bajoran plotline was partly to blame and.