I can't make a comparison, because I have never used Rocksim. But OpenRocket is a pretty amazing free program. I've mostly used it after building a rocket to determine stability and whether nose weight is required, speed off the rod/rail for safe launches, projected altitude, and optimal delay. One field I fly at has a G motor limit and 1,000 foot ceiling. So for some rockets, I've used it to be sure I get close to 1,000 feet but don't go over. And for larger HPR rockets on that field, I've used it to find G motors that will safely lift the rocket and the appropriate delays, which are sometimes surprisingly short.