When comparing 13th Age 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 13th Age is ranked 27th. 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 The game is built for people who've done it before
The world is half built, letting you get in and leave your mark. Each class plays mechanically different than other d20 versions. Skills are placed by backgrounds. It's a short hand and interesting way of playing DnD.
Pro Long lists of skills are gone completely
And covered by backgrounds (much like FATE or Cortex+) so that players aren't searching through lists of skills and what they can do constantly.
Pro Gonzo without the grid
13th Age has some very gonzo fights with party combos, lots of variable powers for monsters and PC's but as a simple set of rules for combat that speed it up without losing a lot of options.
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 Too similar to 4th
Con The Icons and the PC relationships to them can confuse
Can be confusing, especially for new players.
Con It doesn't really do anything new, it just looks pretty
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.