Iron Crown Enterprises (ICE) supports two distinct fantasy role playing engines (Rolemaster and HARP) and two science-fiction role playing engines (HARP-SF and Spacemaster). Each game engine has distinct features and will appeal to different players and different styles of GMing. Rolemaster and Spacemaster will appeal to hardcore players and GMs that want gritty realism and a detailed and flexible gaming experience while HARP and HARP-SF will appeal to fans of high adventure who want excitement and adventure but with simpler rules and faster game-play.
Rolemaster and Spacemaster have much more detail and greater levels of granularity than HARP and HARP-SF. For instance, skills in HARP/HARP SF cost either 2 DPs or 4 DPs, skills in Rolemaster and Spacemaster have a much greater variation allowing for more nuanced professions and more realistic characters.
However HARP and HARP-SF have potentially faster game-play than Rolemaster and Spacemaster. For instance, combat in HARP/HARP-SF is one roll resolution (unless you fumble) whereas combat in Rolemaster/Spacemaster is attack roll then critical roll.
Whereas HARP and HARP-SF are complete games in one/two volumes (SF always takes more room), Rolemaster and Spacemaster can be published in single volumes but there are always sacrifices that have to be made to accomplish this or you need really massive products. HARP and HARP-SF are designed to be expanded with optional extensions, while Rolemaster and Spacemaster are more modular and have more options built into even their core systems.
Rolemaster and Spacemaster are designed to be more realistic with the excitement of the game stemming from that realism. HARP and HARP-SF however will be making real the high adventure of their names.
If you like detail, gritty realism and multiple rule options to craft your own flavour of play, Rolemaster or Spacemaster are the games for you. If however you want a core set of rules that covers all the bases in a single book, and provides comprehensive but swifter, “high adventure” play, HARP and HARP-SF should be your first choice.