I prefer one year leases, but have gone with 6 month leases for people I thought would be good tenants. **I'm assuming you're in Alberta, this only applies in Alberta, and I think you mentioned you were here in another post**
I collect a security deposit equal to one months rent, which gives you some leverage on enforcing the term of the lease. For example, if a tenant does a "midnight move" and you keep the deposit, you should be able to get a new tenant before the one month is up, resulting in no vacancy loss. (Of course, if anything is damaged the deposit could go to that as well, but it helps with their motivation to stay as well, since they want their deposit back)
The biggest advantage to the landlord from a longer lease is that you don't have the hassle, expense, and risk of vacancy associated with trying to find new tenants as often. It doesn't restrict your rent anyway, since increases are allowed in Alberta only once per year.
Regards,
Michael