I'll offer a slight variation on the answers to-date.
Firstly, the decision will be based upon your local market conditions, tenancy law in the jurisdiction, your own preferences and the property in question.
When vacancy is low, the risk to the landlord of a month-to-month tenancy is mitigated. One could even argue that in a market where there is upward pressure on rents, an annual lease may be disadvantageous.
Tenancy law in your jurisdiction will also be a factor. If it is easy for you to terminate a tenancy with a months notice, then month-2-month offers flexibility to both landlord and tenant. If you are in a jurisdiction where termination of a month-to-month tenancy is not easy (hello Ontario), then you may want to take a closer look at any benefit of an annual lease.
Finally, the unit in question is a factor. When we rent furnished rooms, or bachelor/1-bedroom apartments, to students we use month-to-month leases as turn-over is generally quick. We have a couple of larger houses (one student, one for families) for which we use an annual lease as the impact of turnover cannot be justified for a tenancy less than 12-months.