What kind of difference between js Merge and js Bundling for speed optimization.
Minifying is removing unneeded white spaces and newlines, bundling is merging together various resources in one
In terms of speed optimization both techniques should (in theory) improve speed as they are supposed to reduce file size and requests made to the server
The detailed differences between merging/bundling are explained here: https://devdocs.magento.com/guides/v2.3/performance-best-practices/advanced-js-bundling.html#magento-merging-and-bundling
Basically merging creates only one js file while bundling splits the js resources into different bundles (they can't both work at the same time), this is also useful: https://devdocs.magento.com/guides/v2.3/frontend-dev-guide/themes/js-bundling.html