Prices:
For food and litter, petsmart has better prices here... For ex. Nutro foods are usually ~$5 more a bag at petco. Plus, petsmart puts their foods on sale more often.
Toys- similar prices
Selection:
Food/Litter- Petco has more brands of food (the one here at least) than petsmart. i.e. natural balance and castor & pollux, and old mother hubbard (cans only).
petsmart carries blue buffalo and by nature.
toys-my petsmart has a far better selection than the petco here... The only "aisles" of cat stuff at our petco are small, ~4' metal stands with things hanging on either side... cats have about 1 of these with toys on it.
Other animals:
Petsmart for live fish selection (although I find the variety and health similar in both stores)
petsmart also carries more aquarium supplies.
Petco wins for reptiles/herps with a larger live animal selection and more paraphanalia. Our petsmart has only the standard 8 unit cage setup and a few herp supplies.
Small animals: Petco tends to have more varieties of small animals on hand. Petsmart has a better supplies selection here though.
Birds: Petsmart carries more live birds and has more supplies than petco
Dogs: About equal (takes up over half the store in BOTH stores)
Shelter/adoption: Both do adoptions here, and I think both keep animals through the week. Not sure on other details though.
I work at a Petco, and I must say (and all of the associates at my store agree) that Petsmart is definitely cheaper.
However, our Petco has a slightly larger selection than Petsmart, so if you're looking for odd things, you might be more likely to find it at Petco. If you just want food or litter, it's definitely worth going to Petsmart.
I actually don't shop at either. I'm lucky enough to have a Pet Supplies Plus. It is cheaper on everything, and has both the food brands I feed and the litter I use. PetCo dosen't carry Cat Attract Litter, and PetSmart doesn't carry N.B. food.
I have a Petsmart, Petco and Pet Supplies Plus all within a 10 minute drive of me. Don't like the local Pet Supplies Plus - store doesn't seem well organized and there's always a slight odor for some reason.
I have a Petco discount card, and sometimes shop there - they do seem to have more toys, but less selection on other items. Fairly friendly staff.
I admit I'm loyal to my Petsmart - I adopted my two through the partner shelter, they have adoption days frequently during the month for various other shelters, and the store personnel were nothing if not patient and encouraging during my very newby days. Also, they carry Cat Attract litter, while Petco doesn't for whatever reason.
Also, nice return policy at Petsmart, but I believe Petco has a similar policy.
I like Petco and there are often good discounts and coupons to be had. But I've found Petsmart to have cheaper prices and more selection; unfortunately, there seem to be fewer stores so there's not one near me.