@flysnj Hi, to answer your question:
Density - normally forecast is displayed in 3 hour intervals, but in Premium you will get forecast in 1hr steps, which is much more precise.
Quality - in free version, ECMWF is only updated 2 times a day (every 12 hours), in Premium this intervals lowers to 4 times a day (every 6 hours).
Speed - we offer faster delivery of our data.