Where to stay, Baixa or Av Liberdade?

I know its a subjective question but I've never been to Lisbon so starting from cold. Firstly really looking for a good company that lets apartments and secondly what do the different areas have to er. As a guide our likes are really old town, nice restaurants, traditional bars, architecture but would love to hear the experiences of Lisbon from you guys...............cheers MrLob