A shearling will be able to serve more ewes. Also, it is easier to see the quality of a shearling than a ram lamb. I divide my ewes into small groups anyway, each with it's own ram, so I frequently use lambs, often to 'prove' them ie see that they are fertile, and the type of lambs they throw - if they are good, then I might sell them on as proven shearling rams the following year. If you are just breeding up to 10 or 15 ewes for meat lambs, a ram lamb would be fine, and easier to handle.