I haven't bought either in a while, usually only doing it when I brain fart on replenishing my supply from Big Cheese Rodents.
Petco has tended to be cheaper for their Mice on Ice. However, the Mice On Ice tend to also not be individually sealed, and a number of times I've found the product to be freezer burnt. This might be a store thing versus supplier issue. Mice on Ice is Gourmet Rodent if I recall.
As a former PetSmart Pet Care Lead, even with my employee discount it was cheaper (at that time) to get mice from Petco over purchasing Artic Mice. PetSmart also upped the price a number of times on their frozen mice while I was employed there. They (Arctic Mice, not PetSmart) had a
Salmonella outbreak 2 years ago (source:
http://www.tampabay.com/news/enviro...read-salmonella-to-snake-owners-in-18/2180813). The positive for Arctic Mice though, and again I've not purchased them in a while, is that each rodent was individually sealed off from the other. I like this when not buying in bulk, but I buy in bulk so this like only appeals when again I brain fart of re-ordering. Arctic Mice is Reptile Industries, which is Mark and Kim Bell.
In the long run, it may actually be cheaper for you (not sure of your situation on food needs for snake or snakes) to utilize a local/in-state frozen rodent supplier, versus paying retail pricing from Petco or PetSmart. Layne Labs is in Arroyo Grande, CA and I've used them once with a general positive experience. I now use Big Cheese, which is local to me for either being shipped or I can physically pick up at their facility.