annual basis is very risky in qatar. because you are not dealing directly with the land lord. please check qatari law regarding this issue. you have no legal standing, in case of any dispute between the land lord and the agency with whom you are making the contract.
the annual payment trend is increasing in doha from the past few years and people have very bitter experience. recently tenants of complete apartment building were served the notice by court to vacate the building (Gulf News), because of rent dispute between the agency and the land lord.
I will suggest go only for monthly rental.