But they are not completely comparable.
RequireJS is both a module / dependency definition syntax and a bundler. And RequireJS is (AFAIK) the only bundler, which can do the bundling in browser (and work TTW without NodeJS backend server).
Webpack is only a bundler. Webpack works just fine with RequireJS-style definitions. Currently I even rely on our ResourceRegistry definitions when bundling with Webpack.
And webpack may not work at all for pre-building additive bundles in add-ons. @djay asked about, if and how Webpack could be used to build add-on bundle for Plomino. Yes, webpack could be used to build a bundle, which includes all the required Plone JS and Plomino JS.
But I don't know, if it's even possible to build a bundle, which expect Plone logged-in-bundle to be loaded before it (and not duplicate anything in it). And it probably breaks if one tries to include two independently built webpack bundles on the same page.
I prefer Webpack, because it helps me to get work done, but does add an another complex buildout-like tool to learn.