P: Tool presets vs brush presets

Tool presets  apply to every tool, not just tools that use brush tips, and currently can store more information about the brushes than brush presets can. That's the simple explanation.  I'm assuming you're not interested in tool presets for non-brush tools, such as the Rectangle tool or the Type tool, but those are also useful to have a few on hand.

Not everything about the current brush you're holding gets saved with a brush preset. But it all can be saved with a Tool preset. This is especially useful if you customize a Mixer brush in the Options bar and don't want to have to go through all that again, or if it's very important to save the color with the brush.

If you save your heavily customized brushes as Tool presets, you'll always have the precise tool you're looking for. It's not always necessary, or even desirable, to save every setting, but if it is, your choice is often going to be a Tool preset instead of a Brush preset.

Also note that if you're creating brushes to be used in PSE, that program doesn't use Tool presets, or didn't as of the last version. I don't know what the newly released version may do.