My advice would be to take what you can get, when you can get it.
Sure, someone may love your sites so much they'll intentionally rebill month after month, or may forget or die and never cancel their membership. But this is much rarer today than it was some years ago (except for the speed at which people die of course). Today lots of people cancel right away to make sure they're not rebilled, and others don't stick around more than the second month or maybe the third.
The rule I follow is to allow someone to give me as much money as they want to give me, and if someone wants to pay for a year in advance then who am I to tell them I don't want their larger payment.
Just my opinion of course.
