HiFi Simulation’s Active Sky Cloud Art and REX Game Studios’ Soft Clouds are two popular cloud texture products that are fully compatible with P3Dv4 so far.
Although it may not justify to compare the two products head-to-head as AS Cloud Art comes with additional features rather than just pure-enhancement to the cumulus clouds in Soft Clouds, I think it would still be interesting to find out what the two products bring in terms of cumulus clouds improvement only.

