When comparing Monsters & Magic vs Traveller, the Slant community recommends Traveller for most people. In the question“What are the best tabletop RPGs?” Traveller is ranked 7th while Monsters & Magic is ranked 42nd. 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 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.
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 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.
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.