Questions about travel and accommodations in Sifnos and Milos

Do these divisions of time seem about right or should we bias our
schedule towards one island over another?

3 nights on each island is a minimum, do not remove a night on one to assign it to the other.

We are going to Milos for Beaches

Note that in Milos most of the large sandy beaches are located in the west of the island which is an almost uninhabited area (Natura 2000 protected area).
A car is essential and rental companies will only allow you to go there if you rent a 4x4.

The other beaches are impressive because they are made of volcanic rocks but the sandy parts are in the minority.

we were wondering if it was preferable to stay in a hotel/resort or
Airbnb?

Most of the accommodation are run by locals, ranging from studios to large villas with swimming pools or apartments with kitchenettes. It all depends on your budget and preferences in terms of independence.
Going to a hotel is in my opinion without interest unless you are too lazy to prepare your own breakfast every day with that you have bought on site in minimarkets and bakeries.

In Sifnos we are thinking about staying around Appolonia

Apollonia is the capital of the island and where you will find the most businesses, accommodations, boutiques, and restaurants.
It is also the hub for all the bus lines that go to all the other parts of the island.

In addition to Booking, Airbnb and other platforms you can go to:
https://www.roomsinsifnos.com/

which lists most of the accommodations in Sifnos.

We have no idea what area to stay in in Milos

The capital (and the port) is Adamas, right next to it there is the village of Plaka which is interesting. But be aware that you will need a car if you want to go to the beaches. Especially since the bus network in Milos is not that great.
Otherwise you also have the possibility of going to the north of the island to Pollonia which is a small village with a lot of accommodation, one or two beaches and a whole row of restaurants along the coast, rather touristy with even a Greek-Japanese fusion restaurant (!)

Personally I don't like Milos too much, I only spend a little time there and always go to Kimolos which is accessible from Pollonia and which suits me more in terms of tranquility and authenticity.

I prefer Sifnos, its villages, its inhabitants, its beaches, its food and its many hiking trails. https://sifnostrails.com

Some places in Sifnos over the years..

https://photos.app.goo.gl/VzC4AhQsonCjLnkw5

We split 9 days between Sifnos and Milos with 5 on Sifnos as it has more to offer with more villages, beautiful Kastro with the amazing Church of 7 Martyrs just below the village, decent beaches and good selection of shops, markets and tavernas. At the time good bus service all over the island. Middle of island has series of villages you can walk from one to the other with classic Cyclades character.

Milos is best known for it's unique geological formations and lovely beaches. Bus service was very limited. Unique and colorful fishing village of Klima, Ancient Catacombs, amphitheater, and the site where the Venus de Milo was found.

Both islands are worthy of a stay but we found Sifnos had more to offer.

My husband and I stayed in Milos for about 4 nights in 2017. We found it magical. It is small and there is not a lot "to do," but the geological formations along the beaches are spectacular and they glow in the light at certain times of day. There are also a few nice sandy beaches, but and some rocky beaches.

We stayed at Captain Zeppos info@captainzeppos.com in Apollonio. Staying there helped make a great stay for us as Mikhail and his family are wonderful and warm hosts, and served good breakfasts. They were friends with all the restaurateurs in town and we had great meals thanks to their recommendations.

I would say 4 nights is enough for Milos.

This year we plan to go back to Milos for a few nights, then to Sifnos and possibly Felagandros.

Enjoy! These are both good choices, IMHO.

ps you definitely need a car on Milos.

Milos is known most for its geological landscapes formed from volcano rock, ashes and lava. I was a bit disappointed beaches aren’t like those in Naxos. Most accessible beaches are a mere strip of ~ 10 rocky/pebbly sand. The two a bit more expansive are (1) Lagada Beach in front of Hotel Lagada at the western end of the main road in Adamas, and (2) Papikinou Beach on the eastern end of main Adamas road towards the airport. Nevertheless, the geologic formations are glorious and you shouldn't miss.
Many of those formations are accessible by boat only. Few are accessible are by 4x4 SUV only. If taking a boat tour, do not reserve thru tripadvisor, booking, etc. I found them twice the cost of those booked locally. Even at that, there is still a price range depending on boat sizes and tour duration (half day, full day, 6 hours, 8 hours, 9 hours). They fill up fast though, so book fast. There are sailboats and catamarans.
You need to rent a car, which you can pick up at the airport or in Adamas or Pollonia. Streets are narrow with many sharp curves and u-turns. Plaka (capital) and Tripitiki are elevated and Klima (fishing village) are great to watch sunsets from. GPS is a must. Roads are not readily named. If you ask locals, they will give directions by distance, not by street names.
There are many accommodations in Adamas (center) and Pollonia (north). Sarakiniko Rooms are close to Sarakiniko Beach which isn’t quite a beach but are made of beautiful volcanic formations with a strip of shallow beach inlet, with cliffs from which to jump into the water, then you can swim to the cliff wall to climb back up or swim around to the beach inlet.
Sarakiniko Rooms is the closest accommodation to Sarakiniko Beach accessible by walking (10-15 mins) or by vehicle (1-2mins). It feels remote but actually only ~4km from Adamas, hosted by the lovely Margarita. There’s also Hotel Glaronisia with a pool and breakfast buffet, about 500m outside Polonia, hosted by Tzack. Polonia is about 11km from Adamas. Both are ~70 euros. Then there is Olea Bay (>120 euros) which has everything (breakfast and pool) of the previous two have and is about 1.5km west of Adamas. All are very clean and well-kept by friendly, attentive hosts and staff. Note these prices are for last 2 weeks in May.
If you pick up a rental car from the airport, note the 25 euros surcharge if you’ll be returning it before they open at 9am, so they can have a staff there (our flight was at 8am). This is one gripe I have. With incoming and outgoing flights occurring between 8am and 3pm, why don’t they staff the airport according to the flight schedule. I feel it’s another way to tack on the unnecessary surcharge.