Umm, for the sake of the argument, I will assume were talking about both of them in s5. (cause dam, by s7 I'm not sure I want either them to protect me!)
So on one side, we have Buffy, who is debatably at her strongest in s5 (before the depression, and with the big sister "I will do anything for you" mentality). On the other side, you have Willow, who might not be at her strongest in s5, but she's at her sane-est, and she did not even hesitate to confront Glory when it came to protecting Tara.
Assuming that both Buffy and Willow like me enough to actually protect me, I think I would pick Willow. After all, it is true that she was the closest to stalling Glory, she's got the smarts, she's got some mad computer skills, and she managed to keep everyone safe while Buffy broke down and became completely useless. As far as track record goes, hers is better than Buffy's in s5.