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.