If lines are short, the cashier is probably faster. She (or he) can scan things faster, because of experience, and she or he has a bagger to facilitate things. I always go with the cashier if there are no lines.
In BJ stores, you can scan your item with your smart phone, add it to your "cart", then pay for the total bill on your phone with a debit or credit card. Then wheel your stuff out of the store after theexit employee scans the items to see that they agree with the bill shown on your phone, also scanned. You completely miss a waiting line. Much better than any other store.
Scanning the item at its shelf priced location also is ab advantage to see if there is an error.