Both characters grew throughout the show, and both became very likable characters (IMO).
So, I think it's neck and neck. Both characters are 'strong.'
Maybe we should place them both in the same room and see who would win a knife fight. Do you recall the knife fight with Faith and Buffy when they got the knives to each others neck at the same time.
I still need to catch up on my Buffy, but I get the point (pun not intended!)...
Both characters (Jadzia and Kira) were able to handle themselves pretty well; I recall the episode when the treaty with the Klingons was on hiatus, and the Klingons boarded the station after breaking through DS9's defenses. (I think the only one who got knocked to the floor was O'Brien, and an unnamed yellowshirt).