els usually simply define the forward pass from noise to a less noisy sample,
schedulers define the whole denoising process, i.e.:
How many denoising steps?
Stochastic or deterministic?
What algorithm to use to find the denoised sample
They can be quite complex and often define a trade-off between denoising speed and denoising quality.
It is extremely difficult to measure quantitatively which scheduler works best for a given diffusion pipeline,