I have had a small house on the Pelion peninsula for 27 years. It depends what sort of beach you want to be near - on the Gulf side the beaches are more likely to be pebbled/small stones but the sea is calmer and safer for children, timid swimmers - the views are very good. On the Aegean side, there are rock pools, waves, etc - still some pebbly beaches but also some sandy ones. The Gulf side of the peninsula is less steep and less dramatic but very beautiful and with lots of villages worth visiting and good walks. The Aegean side is steeply rocky (forested still) and more "wild" (dramatic scenery). The bottom part of the peninsula is hotter, lower and, to my mind, a less attractive place to stay though you might like to drive down there to have a look around at the villages there.
Presumably you are renting a car but, if not, you should probably stay on the Gulf side (try Kala Nera, which is small enough but has a minimarket, a butcher and a baker and is served by a lot of buses that run between Volos and various Pelion destinations). ALSO, there is a small "mountain" railway that runs every weekend and sometimes every day in the tourist season, running from Ano Lehonia (Gulf side) up to Milies (Milies and Vizitsa are good to visit but particularly well known sights for tourists). The train used to be steam (when I first went to Pelion) but has been converted to diesel, sadly. The rail trip is pretty spectacular.
I don't know what time of year you are visiting as I haven't read your other post yet (which may say) - bear in mind the Pelion will be a little cooler than the Peloponnese, which is a good thing in the summer. There are less foreign tourists (although lots of Dutch people come every year to stay in the campsites) but there are still plenty of Greek holiday makers/day/weekend visitors. Obviously, I think the Pelion would be your best choice (I have visited the Peloponnese as well), especially if you want to go over to Meteora.
