It depends on the store. There is another chain around here were one store is the best chain I've ever seen and another that I wouldn't buy factory sealed food from. Both the same name out front. It depends on the indivual store's manager as much as the staff.
Petsmart (I might be biased though) all-in-all, all the Petcos I went to before I stopped going there there fish had Ich and were in terrible conditions (not to mention the reptiles)....There are a few unknowledgable staff at Petsmarts around me but in general they seem to keep the smart ones there...My Petsmart has alot of intelligent staff and in general will not advice to keep a Oscar in a 55gal or keep a Pacu at all, Pleco in a 10gal etc...Policy is a real killer to both chains IMO/IME
I'm thrown here. Usually Petsmart kicks Petco's ass, and has bigger facilities with more healthy livestock but their staff is hit or miss. My local Petco has actually had a VERY knowledgeable staff lately. So I dunno, I picked petsmart by default.
In my area, I go to Petco for plant supplies & Petsmart for fish supplies. Their inventories are each different & I cant get the same stuff both places.
Petco here is better. It's cheaper and cleaner and each fish tank (where they keep the fish in) is about 12-18 gallons, while the ones in Petsmart is only like 5 gallons. If I buy enough stuff I get things for free that are a good deal, while in Petsmart I just get something like a dog chew toy that I don't need.
The only thing is that Petsmart has a better tank ornament selection.