When comparing Open d6 vs GURPS 4th Edition, the Slant community recommends GURPS 4th Edition for most people. In the question“What are the best tabletop RPGs?” GURPS 4th Edition is ranked 10th while Open d6 is ranked 22nd. The most important reason people chose GURPS 4th Edition is:
GURPS can be as crunchy as you like, with rules available to cover any situation, but at its core, there are only about three rules you need.
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Pros
Pro Includes rulebooks covering the more common genres
Open d6 includes rulebooks for Fantasy, Adventure and Space settings.
Pro No special dice required
The game can be played with common six-sided dice.
Pro A simplified variant, called Mini Six, is available
Mini Six is a fan-made variant of Open d6 that aims to be a distilled version of the Open d6 system.
Pro There are theme-related books available to start various types of games
D6 Space is West End Games old "Star Wars D6" game with all of the licensed Star Wars names removed. It's a stand alone complete game.
D6 Fantasy is a standalone ruleset for running a D&D type game with the D6 system.
D6 Adventure is a standalone ruleset for using the D6 system in a Wild West, spies, or modern adventure game.
All three have supplement books (Spaceship design, magic, aliens, etc).
Pro Each rulebook has a good introductory section
Pro Modular rules
GURPS can be as crunchy as you like, with rules available to cover any situation, but at its core, there are only about three rules you need.
Pro Great sourcebooks
GURPS sourcebooks are famous for being comprehensive guides to the settings/genres they describe, to the degree that even people who don't play GURPS find them useful.
Pro Option for Cinematic Rules
There is an option to make combat far less deadly.
Pro Long character creation creates a bond with your character
Spending a lot of time creating your character also helps with creating a bond with them. This way you will be more encouraged to keep them alive than going around doing dangerous things which would most likely end up killing them. Moreover, In depth character creation flushes out the back stories that enrich the role playing experience. Vampire also has a lengthy character building process for this reason.
Pro Character mortality
In GURPS, characters can be killed by a single blow to the head with a wood plank. For gamers seeking a more "realistic" level of mortality, GURPS is your game.
Cons
Con Complicated magic system
Con Extremely Simulationist
Con Requires too much GM babysitting
Too many options with no setting structure and the freedom to do whatever is a liability, not a perk.
Con Boring rule system
Con Boring rule system
Con Core rules need two books
Con Long character creation
The lion's share of the work in GURPS is front-loaded: characters are built on points, pieced together with attributes, advantages, disadvantages, quirks, skills, powers, spells, cybernetics, and whatever else your game requires. Expect to spend easily an hour or more creating your character.
Con Option paralysis
Sometimes having so many dials to turn is not a good thing. It can be hard to create a game with so many options available.