Depends on the product.

Monthly if I am trying something new. Otherwise, always annual. And even better "lifetime"

Monthly because it’s rare I feel confident enough to pay a year’s worth of a service at first, unless it was dirt cheap, and it gives me as the customer extra insurance that I can stop my subscription without extra headaches compared to annual (can I get a pro-rated refund? Do I have to follow up with you in a year to really make sure I don’t get charged again?)

I usually start out monthly to try out the service. When I decide it's for me, I usually switch to annual so I only have to deal with one invoice rather than 12. You usually get a 17% discount as well (12 months for the price of 10) which is a nice plus.