Bazel is the open sourced version of Blaze, google's internal build system. Buck is Facebook's open source version of their own implementation of a Blaze-like system. Pants was built as an open source implementation of Buck, before Buck was open sourced.
From a bird's eye view, knowing this history of their creators, I would guess that Bazel is more complicated internally, its API has a wider surface area, and ships with many optimizations included for the use case of building extremely large artifacts with many, many dependencies.
Someone who's more familiar with both, feel free to add details or correct my over generalizations! I've used Pants professionally and tried setting up Bazel for personal projects, but found it was too complicated for my needs.