I am using coinbase since day 1
The OP is actually asking about merchant services, that is the customer experience when a merchant accept bitcoin via coinbase verses bitpay.
I have personally only shopped at online stores that accept coinbase and I think their process is very simple. Once I have everything I wish to purchase in my shopping cart, I start the checkout process and select that I wish to pay via bitcoin, I am then taken to a coinbase page where I am given a BTC address and an amount that I need to pay. I pay the amount within 15 minutes and as soon as the TX is propagated I am given credit (even if it takes longer to confirm).