Fast answer
For a low-friction Monopoli beach day, use Cala Porta Vecchia when the swim should stay attached to the old town. Use Porto Ghiacciolo or the Santo Stefano area when the day should feel more scenic. Use Capitolo when sandy beach time, lido structure, and a beach-first rhythm matter more than staying close to the center.
If you only do one thing
For a first trip, keep the first swim simple and choose Cala Porta Vecchia or the nearby south-edge beach hop unless the beach day is the main event. Move to Santo Stefano or Porto Ghiacciolo for a stronger cove setting, and to Capitolo when the group wants a longer sandy-beach day with public/private beach choices kept separate.