Does anyone have any opinions on these islands?

Yes.

Sorry, I couldn't resist.

Ios is a charming island with a small population. It's an island that attracts a large population of 20-somethings and backpackers during the summer months. Drinking to excess seems to be the primary activity. there are a couple of nice beaches.

Paros is a very nice island. It's quieter than some of the other Cyclades although I'm sure it gets crowded during the summertime too. There are many villages and beaches around the island, a few historical sites and a small but interesting archaeological museum.

I spent 4 days there last year with a motor scooter and really enjoyed it.

I haven't been to Naxos in a very long time, but when I was last there I though it was very charming. There are some very nice beaches there, and an efficient bus system services them.

Naxos has great beaches, and Naxos town is larger than Paros town, with several antiquities in the town and in the countryside.

Paros and Naxos are only 30 minutes apart by highspeed ferry, so you don't have to choose between the two - go to both. From Paros you can take a day trip to Antiparos, only 30 min from Parikia. Both Naxos and Paros have good bus service, so it's easy to get to their many beaches and picturesque inland villages.