The advantage of HARP/ RM is the true flexibility of rules and the professions; the professions aren't jails where your creativity lies.
With the system of magic of HARP you don't have 3000 spells like in RM.
The choice of game is up to You. Some prefer RM some HARP. I know that you know it: my choice is clear: HARP.
You can truly play with only the base book to feeling if you are in tune with this game (a complete bestiary, complete magic, equipment list, magical herbs, councels to masters, and so, and so...
For the new material, the staff of creators can answer better that me to You.
But I know that they want edit a book on the evil forces and evil spell casters, called "Something Wicked" but in 2008 apparently (beginning, end?).
The game is complete at large:
- base book
- Monsters: a field guide: a good bestiary (animals, undead and their powers, demons, fiends, and others critters...)
- College of Magic: rules to create Spells/magical items/rituals/runes, new spell casters (elementalist, thaumaturge, necromancer, vivamancer)...
- Grimoire: more and more spells for several mages professions
- Harper's bazaar (1 through 10): more little rules/options, races, professions (paladins, beastmaster), magic items, elementals, spirits with shamans... Harper's are compilated in "Annual Bazaar" (1 to 6
- Harper's codex: 4 new professions (adventurer, mystic, shadowblade, druid), more spells...
- Hack'N'Slash: a new system for combats (I love it) ultra/mega/horrendous
- martial law: new armours, weapons, wound descriptions, skills and talents related to combats.
It's all my books and we play very fun RPG evenings.