Kendra was taken out by Drusilla, for heaven's sake. She never vanquished any major foe, had no imagination and was a completely by-the-book Slayer. Buffy, on the other hand, repeatedly defended Sunnydale and her friends against multiple major threats and devised ingenious ways of outsmarting her enemies. On a personal level, Buffy was funnier and warmer, less reserved and close-minded about things. She questioned authority, didn't let pointless rules and regulations get in the way of doing what she felt was right. How would Kendra have handled the Watcher's Council in Season 5? We all know from experience that they're all basically a bunch of self-serving, power-crazed bureaucrats. Buffy challenged them and ultimately won. She saw the cracks in the system and dealt with them. Kendra would have deferred entirely to any authority figure that came her way. The Slayer needs to be able to lead. Buffy gradually developed that ability. I doubt Kendra ever would have had the gusto to lead people into combat in the way Buffy did in Seasons 5 and 7. Plus Buffy had her friends to rely on. They enhanced her strength. Without them, she would not have survived all seven seasons. Kendra, by contrast, was all alone. She had no friends to speak of. As was mentioned in the series, a Slayer with friends is a completely different animal from a Slayer with none.
And if it came down to a one-on-one fight, Buffy would win because, despite Kendra's skillful fighting style, Buffy is the sort of Slayer who would never give up, regardless of how hopeless the outcome seemed. Put simply, she doesn't know the meaning of the word 'defeat' (the only exception being late in Season 7 when she does temporarily enter a state of self-pity from which Spike is forced to bring her out). Basically, if Buffy could defeat Angelus by Season 2, then she could definitely take Kendra (not to mention the fact that Kendra hadn't been a Slayer for as long as Buffy anyway. And there's a huge difference between a lifetime of training to fight vampires and actually fighting them).
