In python formulae, what is the difference between depends_on and a resource for the same PyPI package?

[unitof]

What (if any) is the difference in a formula between, say

depends_on "cython"

and

resource "cython" do
  url "https://files.pythonhosted.org/packages/0e/17/c5b026cea7a634ee3b8950a7be16aaa49deeb3b9824ba5e81c13ac26f3c4/Cython-3.0.9.tar.gz"
  sha256 "a2d354f059d1f055d34cfaa62c5b68bc78ac2ceab6407148d47fb508cf3ba4f3"
end

for a formula which depends on cython from PyPI?

Is it always redundant to have both? If so, which is preferred?

(Context: I am working on a manim formula and trying to clean up my resources/dependencies.)

Update/Example

In testing, I've discovered using depends_on "pygments" in my formula does not work. The formula builds but fails brew test manim with:

==> Testing manim
==> /opt/homebrew/Cellar/manim/HEAD-9f3d486/bin/manim -ql /private/tmp/manim-test-20240319-38046-b2toy4/testscene.py CreateCircle
Last 15 lines from /Users/jacob/Library/Logs/Homebrew/manim/test.01.manim:

Traceback (most recent call last):
  File "/opt/homebrew/Cellar/manim/HEAD-9f3d486/bin/manim", line 5, in <module>
    from manim.__main__ import main
  File "/opt/homebrew/Cellar/manim/HEAD-9f3d486/libexec/lib/python3.12/site-packages/manim/__init__.py", line 13, in <module>
    from ._config import *
  File "/opt/homebrew/Cellar/manim/HEAD-9f3d486/libexec/lib/python3.12/site-packages/manim/_config/__init__.py", line 10, in <module>
    from .logger_utils import make_logger
  File "/opt/homebrew/Cellar/manim/HEAD-9f3d486/libexec/lib/python3.12/site-packages/manim/_config/logger_utils.py", line 23, in <module>
    from rich.logging import RichHandler
  File "/opt/homebrew/Cellar/manim/HEAD-9f3d486/libexec/lib/python3.12/site-packages/rich/logging.py", line 15, in <module>
    from .traceback import Traceback
  File "/opt/homebrew/Cellar/manim/HEAD-9f3d486/libexec/lib/python3.12/site-packages/rich/traceback.py", line 23, in <module>
    from pygments.lexers import guess_lexer_for_filename
ModuleNotFoundError: No module named 'pygments'
Error: manim: failed

However, removing depends_on "pygments" and switching to a resource block

resource "pygments" do
  url "https://files.pythonhosted.org/packages/55/59/8bccf4157baf25e4aa5a0bb7fa3ba8600907de105ebc22b0c78cfbf6f565/pygments-2.17.2.tar.gz"
  sha256 "da46cec9fd2de5be3a8a784f434e4c4ab670b4ff54d605c4c2717e9d49c4c367"
end

builds and passes brew test manim. Both the homebrew & PyPI versions of pygments are presently identical: 2.17.2.

[SMillerDev]

Is it always redundant to have both? If so, which is preferred?

Yes, since they represent the same dependency. The depends_on is preferred since it will automatically allow updates without bumping the formula.
Homebrew as a project is reducing the duplication between python resources and formulae though, by removing formulae that do not provide much benefit over resources: Homebrew/homebrew-core#157500

[unitof]

Thanks! So would the issue I ran into (depends_on "pygments" working while resource "pygments" fails in the manim formula) represent a bug with either pygments or its formula?

Or, are there valid circumstances where the resource is required over the Homebrew formula?