What's the advantage of bitpay over coinbase for a merchant solution?
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.