Fast answer
Use the old town when evenings on foot are the trip's fixed point. Use Capitolo only when repeated beach time is the reason for staying in Monopoli. Use Cala Porta Vecchia as the compromise when the trip wants old-town access and a simple town swim.
If you only do one thing
For a first Monopoli stay, the old town or harbor edge is usually the safer default unless the group is explicitly beach-led. Capitolo is stronger for sandy beach days and lido rhythm, but weaker for spontaneous old-town evenings.