Although Petco is closer to me geographically, I prefer Petsmart by a long shot. While i was getting Feliway plugins every month, I hated paying full price. Both places have it cheaper online, BUT Petco does not honor their website prices in store. petsmart does. I used to bring in a printout of the page for Feliway and hand it to the cashier. Honored every time, even different stores.
I like their selection, prices, everything. Even our shelter, where i volunteer, likes them. We have some adoption cages there, so some of our adoptables get even more visibility. Their staff seem to be more attentive, too.
Petsmart, as much as possible.