Petsmart seems to have a little better selection, and better prices on dry goods.
Most of the people who work in the fish department in these stores don't know much about fish. Around here, they are usually 15-16 year olds just wanting some bucks.
I received some quite bad advice when I was new to the hobby and I picked Petsmart to get me started. I bought a 28 gallon tank. They told me I could put water in it and get everything going that night, then the next day, I could fill it with fish. Well, I did what they said, and, of course, most of those fish died.
I did buy an Oscar at Petsmart the other night, and the girl made sure that I had a large enough tank for him, so I appreciated that.