I loved them both, but I followed Buffy all the way through. I stopped watching Charmed before it ended. It really went downhill in the latter seasons IMO. Buffy went out on a high, so it gets my vote.
Both are great TV shows. Full of entertaining and powerful female figures.
But Buffy was smarter, more experimental and more iconic.
Also, I can't watch Charmed past season 4 while I can watch every seasons of Buffy without any problems.