Oh... congrats on the great news! I wish you a wonderful life together!
All three islands are amazing. I've been to all of them and I'd say that Milos is my favourite. Plenty of beaches and things to do, not as expensive as Santorini and Mykonos. Milos boasts some of the best beaches in Greece... It is renowned as the "couple island", at least that is what the locals told me when I went there. I truly had a great time, loved every part of it! Santorini is great for 2-3 days. If you are staying longer I suggest you try Milos or Mykonos since you'll have more things to do. In any case, you can try Greek Island Hopping and see more islands by spending 2-3 nights on each one if you are looking for a more adventurous summer. Paros, Naxos, Anafi, Ios, Mykonos, Amorgos, Santorini, Serifos, Sifnos, Milos can be easily combined.
Anafi and Amorgos are great for relaxation. Paros, Ios, Mykonos, Santorini have a vivid nightlife. Milos has the most beautiful beaches for sure, more than 70! :) Try googling photos from Kleftiko Beach, Sarakiniko Beach and Tsigrado Beach, you'll see what I mean... For greek island hopping you might wanna look here, check the ferries section.