How can I make an RPM depend on package a OR package b?

If packages don't have common Provide, and you don't want to rebuild and maintain forks of that packages, but they both do provide same file, you can depend on a file. For example:

Requires: /usr/bin/pear

That way your package won't matter if pear binary is provided by php-pear or php53-pear.

The standard way to do this is via virtual provides: the providing RPMs each state that they provide a virtual package, and the dependent RPM requires that virtual package. So, in your example, vim and emacs both Provides: text-editor, and your package would Requires: text-editor.

If the packages that you're depending on don't provide an appropriate virtual package, you could probably hack around this by making up your own. Make dummy my-package-vim-mode and my-package-emacs-mode RPMs that Requires: vim and Requires: emacs, respectively, and Provides: my-package-text-editor, then have your package Requires: my-package-text-editor.