Home Depot. In the last 4 years, building our home & then working bit by bit to upgrade it, we've spent probably $45,000 at Home Depot and maybe $5,000 at Lowe's. Home Depot has a friendlier feel to it, while at Lowe's it always feels like they're expecting me to try to steal something. Not the staff (they're great), but the store entrance/exit layout.
I'm glad Lowe's is there, though. Every once in a while, they have just what we need when HD doesn't. Our clawfoot bathtub and both toilets came from Lowe's for instance.