I personally use only live plants. I feel safer with real plants, because many of the fake plants have sharp edges which may injure an animal. I prefer to keep my tanks as natural as possible, and I don't really like the appearance of fake plants. I also like real plants because of more useful purposes, such as using some ammonia and nitrates.
In my opinion, live plants are definately better for breeding because most newts that will lay eggs on plants like to fold the leaves over the eggs. I can see how this may work on silk plants, but not a chance with the plastic ones. Also, with live plants such as elodea/egeria, you can simply break pieces off of the plants and move them to a tank where they won't be eaten while the plant continues to grow-try that with plastic plants.
Live plants will also continue to grow and spread to create more hiding spots and increase the functionality of them. I've seen tons of plastic plants, but have never seen any of them grow.
~Aaron