For beaches
Naxos has the edge if beaches drive the decision. Its west coast includes long sandy stretches such as Agios Prokopios and Agia Anna. Paros still has plenty of excellent beach days, but its appeal is more evenly split between beaches, towns and evenings.
For evenings
Paros has the clearer edge for a social evening atmosphere. Naoussa in particular combines restaurants, bars and a photogenic harbour. Naxos Chora is far from sleepy, but the overall island tends to feel more spread out and relaxed.
Without a car
Don't choose the island first — choose the base. A well-located stay near Chora, Naoussa, Parikia or a serviced beach area can matter more than whether you picked Paros or Naxos. Check walking distances and transport before booking the hotel.
For a 7-night trip
One island is calmer; two islands give you contrast. Seven nights is enough to consider a 3+4 split, but it adds a hotel change and ferry. Put both versions next to each other before deciding whether the extra movement is worth it.