I've stayed at, value, moderates and deluxes and all in all, I think moderates offer the best bang for your buck.
What I and my kids like like about the moderates are the pool - typically moderates offer a larger pool, more pools, and a slide. Take the Carribean for instance, you have a local pool near your building and then the larger pool with a slide.
What I like being a family of four, is that we have two queen beds, my kids are not getting any smaller and two larger sized beds is definitely a plus.
With that said, what I like about the value resorts is the cost, its very inexpensive, and if I was being honest, I'd admit that our time at the resort (vs. the parks) is minimal. If I can accept the slightly cramped sleeping conditions, then the value resorts make sense.
I really enjoyed my time at the Contemporary, and it was litterly a bucket list item. I always wanted to stay at the Contemporary. My wife feels the same way at the Poly and so this time we're staying there, but I'll be honest, I think the moderates offer a better deal. The one thing the contemporary has is its location. Walking distance to MK, and its on the monorail line. The Poly is similar, walking distance to the TTC so you can take the monorail to Epoc, and the resort is on the monorail line.