I can only provide some thoughts about Uster, as I'm not at all familiar with Baden or Dielsdorf. We live in the village next to Uster, and so I spend quite a bit of time there for various things. I personally find Uster to be a very charming little city. Not too small, not too big. It does have at least one direct train to Oerlikon, which takes about 15 minutes. A few other things that I like about it:

1. There is a nice beach and swimming pool area in Uster (Uster Strandbad), on the shore of Lake Greifensee. It's great for kids, with sandboxes, kids pool, playground, etc.
2. There is an English Club in Uster called "WAC" that is for English-speaking expats and their kids. It's a great organization with a lot of fun activities for kids as well as grown-ups. I believe it also has a preschool. It's a good way to make new friends and not have problems with language barriers.
3. As someone else had mentioned, there is a hospital in Uster. I've had positive and negative experiences with it, to be honest. But overall, more positive than negative. I have never seen a long wait in the emergency room there.
4. There is also a large indoor swimming pool there.
5. You won't have to travel far to be in nature, as Uster is surrounded by forests and the countryside (with paths weaved through them) as well as the nice path that goes around the lake.

I'm sure I could probably think of more things to list if I put my mind to it, but for now, that is what's popped into my head.