They're both alternative build tools

Rollup is very similar to Webpack but is architectually different to allow it to optimise assets further and faster.

Brunch has actually been around since before Grunt, it's just not marketed. It works in very simple fashion, just install plugins via npm and start using them, no need to edit configuration files (unless you have very specific requirements).

I would consider Rollup as the new hotness, building upon the success of other tools and starting from a better position to create an optimised tool.

Brunch is a long time standing tool that does a job very well and if your project doesn't require any special configuration or architecture it's so easy to use (it can be configured though).