Until about two years ago Guitar Center was my central source of music gear. Then I found Sam Ash through a friend and came to fall in love with their store. Their customer service is great, always ready and willing to help with anything.
I can't say I've completely stop going to Guitar Center though, if I'm shopping for gear and can't find it at Sam Ash I'll check out GC. It's not that Sam Ash couldn't order it in for me, but I kind of like getting stuff now. But that's my little character flaw, not theirs.
Oh, and one little thing that Sam Ash definitely has the upper hand on is that they sell band and orchestra instruments for some of us band nerds.