By far, i trust PetCo for live plants and bettas. But not live plants to add to my betta tanks, just my garden tanks. They still use snails, like petsmart in their plant tanks, so chances are, when you buy a plant or two, u'll get snails too.
As to taking care of their fish, PetCo rocks. They actively change the water daily, remove diseased, infected, and dead bettas from the shelves, and never let the bettas see each other that much (unless they are veil tails). They love to stack those for some reason.
Equipment wise, petsmart is more like the walmart of fish supplies. PetCo has the high end aqueon setups. PetSmart has the 5G $27.99 tank setups.
As a previous poster said, medications, tank treatments, etc, are easilly found at petsmart vs petco; but then PetCo has the best frozen foods, and live foods. They even have (depending on your location) live ants.
As to WalMart: Say away from these people, except for maybe the tanks. Their decorations are rarely marked safe for anything other than garden kits, and their foods, treaments, etc, are so out of date it's a shame. They also do not take care of their bettas. Leaving them in MB Blue bowls, where they can see each other, and the poor things die too quickly.