This is just based on my really minimal understanding of them from years ago. Fabric is much more lightweight than Forge. Forge (at least for a while) patched everything everytime you loaded up the game. I believe Fabric was made to load quicker. Forge may be different now.
Modded Minecraft
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forge has a well known and established API for a lot of common things while fabric doesn't offer one OOTB so you'll have to fall back to mixins to modify minecraft's bytecode (not source code) in a lot of cases. fabric is a lot simpler in it's design and is a lot less performance intensive; but also lacks a lot of the QoL features a stable API provides.
my recommendation is to try both and get a feel for what you like best, do you rather spend ages looking at the forge documentation or prod at a somewhat translucent box in the hopes of making sense of a vague mixin error.
Hmm... so Forge is more focused on having a centralized location (the API) to interact with the game, whereas Fabric gives the mod maker more control over the game without needing to depend on an API having the functionality required?