Breck's parks are longer, but skinnier, so you can hit more features per run. Breck's features are very fun, smooth, and trickable. As far as rails go, the ones at Breck are definetely much easier than the ones at Keystone. I'm not a very good park rider, but I will try any of the rails at Breck, including the ones in Freeway. The jumps at Breck are also great, very smooth, fun, and not too scary to try new tricks on.
Keystone has wider parks that are shorter. You don't hit as many features per run, but both parks probably have a similar number of features in them. The rails at Keystone are much harder than the ones at Breck. I will not even attempt a lot of the rails in Keystone's park because they have some pretty crazy ones. The rails are not as trickable, and on many of the rails, it will be difficult just to clean the entire rail. They have much more creative rails, and many urban style setups. If you are a very high skill rail rider, you will love Keystone. I don't ever really hit their jumps, but they do have decent jumps of all sizes, but I like Brecks jumps better.
Either way, both parks are pretty fun. The resorts are so close to each other that if your in the area for a couple of days, you really should just check out both. You don't even need a car to get from Breck or Frisco to Keystone, you can take a free bus.