both really...i haven't had any problems with either. But I've dealt more with Sweetwater's customer service (when they were trying to pitch an HD unit to me)
Personally I like ZZounds more...but that's because their site it a little more informative about the products than Sweetwater. But some things can only be found through Sweetwater while others ZZounds carry (like if you're looking for Behringer, sweetwater doesn't carry them).