Paying monthly is preferrable for individuals, because they often do not know in January whether they are going to use this service in June.
However, serious companies prefer annual payments. Service upgrades/downgrades are made within 5-10 minutes any moment you like. Payments are usually computed automatically without any issues.
Therefore, the question is only psychological, not technical. Do you prefer paying often and with smaller portions or issuing payment once a year and forgetting about it till the next year? That is all.