Which is better: A lease or a month-to-month agreement? That depends.
A lease for a fixed term (such as 6 months or a year) locks both parties in for that period of time. The landlord may not evict (unless the tenant misbehaves by failing to pay the rent, wrecking the place, etc.) or raise the rent, and the tenant is responsible for the rent for the entire period. This might be good for the landlord and not so good for the tenant in an area where rents are not rising anyway and/or where there is a "low season" when it is difficult to find tenants (such as a university or resort community).
A month-to-month rental agreement gives both parties more flexibility. Either may terminate the agreement by giving written notice (usually at least 30 days) - unless a local rent control law allows eviction only for "good cause." The landlord may raise the rent at any time simply by giving a similar notice. This flexibility is good for landlords in a rising rental market, and good for a tenant whose plans for the future are tenuous.
Whichever you choose - lease or month-to-month agreement - make sure you put it in writing, in order to make the rights and duties of each party clear, and in order to minimize the likelihood of later disputes over what was agreed to. It's a good idea to have a lawyer help you write up a standard form - adapted to your needs - to use with most or all of your tenants.GotTrouble.com