Fast answer
For most first Monopoli trips, start with the old town or the Cala Porta Vecchia south edge, then move outward only when beach time, countryside space, or rail friction is the stronger constraint.
If you only do one thing
Choose Centro Storico or Porto Vecchio when dinners, evening walks, and a compact first arrival matter more than car convenience. Choose Cala Porta Vecchia south edge when the same trip also wants the simplest in-town swim.