When comparing Dungeons & Dragons v3.5 / Pathfinder RPG vs Monsters & Magic, 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 Monsters & Magic is ranked 42nd. 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 The Effect Engine
The game uses a single core mechanic, which is flexible enough to cover a wide variety of situations. This helps GMs immensely when characters want to do things that aren't already covered in the system.
The mechanic itself works thusly: all rolls are 3d6 + modifiers minus difficulty. The result is how many effect points the player can apply to the action -- or, if negative, how many consequence points their opponent can apply to a reaction. Dealing damage is one use of effect points, but there's a limit on how much can be dealt at once, so additional effects (e.g. distracting, disarming, unmounting, etc.) see constant play instead of only being used rarely or by super-specialists. This also means failure is interesting, since it results in consequences!
This mechanic is used to do everything, and the game essentially consists of nothing but examples of how to use it.
Pro Interesting combat
Attacks generate effect points, which you spend to do things to your opponent. However, the amount of damage you can do is limited, which forces players to do other things... and the effect system is broad enough that these can be things like cutting off a carrion crawler's tentacles, knocking an opponent's sword out of their hand, and so on. Failures on attack generate consequence points as well, which your opponent can use to do things to you.
Pro Mental defense and hit points
Characters have a "Mental Defense" score and "Mental Hit Points". These are used to help provide a unified system for handling social encounters and mental attacks via magic.
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 Not currently available in print
Is only available in PDF.
Con Few monsters & spells provided
The included rules have only about 20 monsters and 1st and 2nd-level spells. Third party publishers have supplements that provide more, or you can use ones from old D&D.