I'm trying to figure out why a merchant would ever choose bitpay over coinbase as an integration solution. It seems they offer very similar services (accept bitcoin payments, exchange for dollars automatically daily and deposit into your bank account), yet BitPay charges 1% and Coinbase nothing.
I would encourage you to look at the customer reviews of each service. BitPay is definitely a stronger and more reliable platform, and is fully "on-blockchain" and compatible with all of the bitcoin wallets. For a low-volume use case, like a few orders per day, probably either service will work. For higher volumes, BitPay is more business-grade, with features like Quickbooks import.