And, even if you prepay, you can still cancel and get a full refund. Even if you know you'll only use the server for 1 month, prepaying for 2 years is a smart move!
Keep in mind that if you prepay for 2 years, use the server, and cancel before the 2 years ends, you're refunded the amount paid minus the normal monthly charge times the number of months you used the service. The discount gets wiped away, at least it did for me.