It all depends. If you have a good relationship with your provider then I would pay per month faster than if it were not the case.
Lets say the money does not get wired and the mail/letter that asks you to wire the money gets lost. If you have only a platonic relationship with your provider they will cut you off. If you have a somewhat closer relationship to your provider chances are high he would try to reach you in any way he can.
Another matter is reliability. Lets assume that you have a provider with a 100% spotless track record. In this case I would not mind paying per year. Lets say you have a less reliable provider but you would not like to transfer everything. The least you can do is paying per month. If things go south you can always spent you money for the remaining of the year elsewhere.
Finally, like it was mentioned somewhere above, you have the issue of the size of investment. If the value for the service or product is really low people are faster inclined to pay per year than in the case of a large sum.
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