Franco Pasut has an instructive post on how to diff files with Vim and Emacs. Both editors have very nice built-in functions to make diffing files easy. I’ve used them both and can attest to their utility.
Pasut’s post, Notes on resolving differences between two documents with the built-in resources of Vim and Emacs, isn’t a comparison and doesn’t make any value judgments. Rather, it merely shows how to use the two systems. I haven’t used Vimdiff in years but my memory is that it was easier to use than Emacs’ Ediff system.
Regardless of their relative merits, both are easy to use and very powerful. To see this, think about how you would reconcile two files with just the standard Diff program and some random editor. With Vimdiff and Ediff, you see the differences one by one and can choose which variant to keep. With Ediff, you can even edit the file directly.
If you’re not familiar with the two systems, take a look at Pasut’s post. It’s short and tells you more than enough to use either efficiently.