I've been to Folegandros, Sifnos, Milos in different years, in that order, with Milos most recent (2005). I loved Milos (and the reason that I'm writing on this forum right now is to get over my disappointment for just cancelling my upcoming trip to Krabi, Thailand due to work).
For me, Milos definitely (you can easily stay for a week) and Sifnos as a second choice. Both islands will be quiet when you go.
- I love the sea water and Milos has incredible beaches (not only because of the water quality but because of the landscape/color/backdrop as well). Some of them are not easily accessible (steep path leading down, etc), but if you are relatively fit you'll have no problem. Best way to get around is rental car, easy, no major traffic, free parking lots in center of Adamantas (the port - which I recommend as your base), ok roads (don't get scared by the gravel stretches, they are ok). With a car you can easily do 2 beaches on a single day.
- I liked the food (esp. seafood). I think it's one of those few Cyclades islands where you can still find fresh fish (variety AND quality) every day. For example, not the case in Folegandros (or Mykonos where a lot of fish comes from greek+moroccan fisheries). You'll find good food in Plaka and on a few beaches (Provatas, Palaiochori), (A)pollonia for fish, and for the ultimate ouzo+mezze head to Mandrakia (tiny community on the north side).
- Top Tip: make sure you take the day trip with captain Nikolas on his catamaran. Can't miss it, it's docked on the main esplanade in Adamantas. Ask for captain Nikolas with the catamaran. Don't miss this trip, so book as soon as you arrive.
- We stayed at Villa Helios Apartments. It's very clean, spacious, well appointed and located, with a view of the port. It's run by an Austrian lady called Beatrice. We really liked the place (and value) and Beatrice is very helpful without being overbearing. Keep in mind that Milos made it on the greek tourist map recently (has a big mining economy) and accomodation was a problem.
- Having seen the beaches/water of Milos, you'll be somewhat disappointed here. Among our group of 3 and all the beaches on the island, we voted only 2 as good (Chrissopigi and Vathy). But the island is famous for its Chora (Apollonia) and the views from it. It's essentially 5 small villages that are almost connected. Definitely stay in Apollonia and take the bus to the beaches (or rent a car - roads are better than Milos). On a clear day, you can see other islands in the distance. Many churches, nice walks, more "color" and architectural appeal (and to many Athenians more stylish) than the Milos villages.
- Quite good food (with some well-known dishes/recipes in Greece). For most, the food is much better here than Milos. You will find a good selection of restaurants in Apollonia and the adjacent villages (particularly one of them whose name I forget). There's also some decent nightlife here (compared to Milos where it's very limited), with a few bars for a quiet drink after dinner (nothing crazy).
- Difficult to get to given your limited time (easier to fly to Santorini and then take the boat up to Folegandros).
- Beaches are not that nice (we found tar on some of them), but the water is great, the nicer ones accessible only by boat.
- Chora (the main village) has quite impressive views, sitting on top of a cliff ("caldera").
To sum up: Milos and Sifnos (although doing both in a week may be a bit tight). Milos for the beaches and landscape (and seafood), Sifnos for the Chora and food. Best transportation option: take the one-way morning flight from Athens to Milos (book it now as the prop planes are really small and fill up quickly - you'll enjoy the ride and the short runway ;) and take the fast ferry from Milos to Sifnos to Athens.