+1 to Petco not Petsmart for the sale. A Tetra brand tank bought in a pet store, Petco or Petsmart should all be the same tank so for that, price becomes more the issue than the item itself.
As for the store in general, it's hit or miss as to the staff's abilities and knowledge. I've read many on this site who say their stores have excellent personnel. My "local" store ( about 50 miles away is the closest to me) has had a higher turnover of staff and I know why ( They weren't very knowledgeable about their livestock.) So I would say" Trust, but verify" for anything you are told. Don't fall victim to impulse buying. THAT kills more fish and pets than almost anything else.
My first choice is always a local Mom & Pop pet store. While you may pay a couple extra pennies for your items, you are paying for their knowledge and expertise which, in the long run, will save you much more than what you would by shopping the discount stores.
If you are new to the hobby, you may want to stay away from used equipment outside of a pet shop as you don't have the experience to know what you are getting. You don't always get perfectly working equipment in these type purchases but how would you know?
Hope this helps