If you accept small payments regularly and don't operate with significant amount of BTC then bitpay or coinpayments is ok for you, but if you need to keep a lot of btc and be sure that transactions you observe are real you need a full node, maybe even several of them.
I don't think using and preferring payment processors has anything to do with the amount, or at least it is not the biggest reason. It is all about whether the merchant wants to receive bitcoin or fiat while the user still pays in bitcoin. Due to volatility of bitcoin price those who start accepting bitcoin prefer receiving the payment in fiat.