When comparing Dungeons & Dragons v3.5 / Pathfinder RPG vs Heroes Against Darkness, the Slant community recommends Dungeons & Dragons v3.5 / Pathfinder RPG for most people. In the question“What are the best tabletop RPGs?” Dungeons & Dragons v3.5 / Pathfinder RPG is ranked 29th while Heroes Against Darkness is ranked 33rd. The most important reason people chose Dungeons & Dragons v3.5 / Pathfinder RPG is:
The D&D v3.x family was published under an Open Game License which encouraged third-party material. Two editions later, there is still a thriving market creating new 3.5-compatible resources.
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Pros
Pro Enormous pool of source material
The D&D v3.x family was published under an Open Game License which encouraged third-party material. Two editions later, there is still a thriving market creating new 3.5-compatible resources.
Pro Actively supported via Pathfinder RPG
When Wizards of the Coast discontinued D&D v3.5 development in favor of 4th Edition, Paizo took advantage of the Open Gaming License to publish an updated revision of the 3.x rules under the title Pathfinder RPG, which is still their flagship product. Notably, Pathfinder RPG has consistently outsold 4th Edition.
Pro Modular system encourages creativity
The D&D v3.x family is designed around building characters, monsters, magic items, etc., from small simple pieces. For each piece, any of countless others can be substituted, allowing near-infinite customization and variety.
Pro Many easy to comprehend books and other resources
Pro Perfect balance of DM storytelling and deeply granular rules.
All of the deep-dive rulebooks provide the option for endless complexity - go as far as you want, and you'll never run out of opportunities. However, the basic mechanic of "roll a d20 and add your bonus" is simple enough that the DM can tell stories, create worlds, and build adventures with their party.
3.5e is the perfect edition.
Pro Focuses on simplicity
The game uses a single "d20 + modifier ≥ target" mechanic for everything. Characters can be created in less than 5 minutes. The writing is concise and clear.
Pro Deep, without being cluttered
A lot of meta-tools such as templates, etc, that make combat encounters incredibly simple to construct. Great tools for creature-building. A lot of good information on world-building. You can make any type of game efficiently, from hack-and-slash to role-playing. If you have played in a D&D campaign, this is a great choice for your first outing as a GM.
Cons
Con Encourages character optimization outside of play
With so many options available to the player, and the almost unlimited ability to combine them, certain "character builds" are patently superior to others. Players who spend a lot of personal time poring over the rules can often create characters so powerful that players who don't optimize feel useless in games with those who do.
Con Casters are significantly more powerful than non-casters
Con Detailed simulation slows down gameplay
Due to countless situational modifiers, players and DM alike have to put a great deal of thought into decisions about position, order of actions, etc., to the point that six seconds (one combat round) of "in-game time" frequently takes half an hour or more of play time.
Con Very easy to build broken (overpowered / underpowered) characters
Con Requries prior knowledge of RPGs to completely understand
Some knowledge will be required to understand the rules as there are some unexplained conventions that are presumed to be understood.