When comparing Dungeons & Dragons v3.5 / Pathfinder RPG vs Traveller, the Slant community recommends Traveller for most people. In the question“What are the best tabletop RPGs?” Traveller is ranked 7th while Dungeons & Dragons v3.5 / Pathfinder RPG is ranked 29th. The most important reason people chose Traveller is:
No magic, no fantasy conventions. This is actually science fiction.
Ranked in these QuestionsQuestion Ranking
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 It's actually science fiction
No magic, no fantasy conventions. This is actually science fiction.
Pro Expansive library of reference materials
Pro Diversity in setting and thematic complexity
Traveller is one of the oldest science fiction roleplaying games, and has a default setting that is even older. As such, there is a lot of cumulative depth that has been developed over the years. The current version of the rules (Mongoose Traveller 2nd edition) is very accessible, however, and allows new players to approach the game without a heavy load of baggage. Anybody who has seen an episode of Firefly, or any other typical space travelling fiction, will immediately understand the tropes. From the basis of its simple system, lots of interesting sci-fi ideas can be developed and there is a lot of material to use as inspiration.
Pro Smooth, fast gameplay
Pro Adaptable
Thanks to various reference materials over the years, and numerous updates, it is possible to run almost any science fiction setting you want, from 1970's space opera, to cyberpunk and transhuman games (thanks to settings like Mindjammer and the addition of cybernetics in recent editions).
Pro Games-within-a-game
Traveller was, arguably, the original 'tool box' game, utilizing tables, dicerolling and design judgment, to generate things such as aliens, planets, star systems, economies, weapons, vehicles, spacecraft and characters. Character generation is a unique experience of taking risks to develop a lifepath history through career development. A bad roll could lead to unfortunate events, injury, prison or even death, while good rolls can lead to boons in skills, resources, contacts and reputation. It's actually fun to generate characters before you even begin play.
Pro No zero to hero
Gets away from the character leveling motif so you can just focus on role playing. Very much the story of typical people doing stupendous things.
Pro Makes complex concepts easy
Starship combat can be a bear in many games. The current incarnation of Traveller has reduced it to a simple, fast, and easy to use game within the game that makes for fun and realistic (enough) game play that doesn't take away from the experience. The vehicle combat system and other rules subsystems work equally well. None are perfect but they enhance the flow of a good game which is what most players want more than anything.
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 Retro future basis can get in the way
It's 1970's sci-fi. So some unexpected modern motifs aren't well represented (transhumanism, wireless tech, internet, etc). There are, in modern editions of the game, plenty of examples of these, however.
Con All editions not created equal
Because of its numerous editions and rulesets, finding a proper entry point can be confusing for newcomers.
Con No character development
Although this CAN be a plus, as you know what your players can/can't do and it doesn't change. Plus its easy to mod it if you want it.