Most processors (whether they be an Internet Payment Service Provider (IPSP) like (some versions of) PayPal / 2CO; an electronic payment gateway (LinkPoint / YourPay, Integrated Internet Payments (IIP), Yahoo!, Quantum, Authorize.net / Cybersource; or a transaction provider like First Data or Nova) will experience some downtime from time to time.
They have to upload changes from the card associations and during this maintenance, if all goes well, nothing will go down. But if something were to happen, then you might have downtime. Most of them have scores of servers to handle the transactions per second: for example, the gift card platform hit over 900 transactions per second (TPS) a couple of weeks before Christmas).
For the most part though, the processors won't go down because most of them support the Fortune 500 companies like Wal-Mart, Home Depot, etc and a lot of executives will usually be woke up when this happens
As to the payment methods, our company do not accept PAYPAL because it's not convenience for chinese especially running wholesale business. But we offer other payment methods for your choice. In my opinion, credit card would be very safe and convenience.