Ocmod is a new standard of modifications introduced in Opencart 2. It allows to upload files in the admin back-end easily. OCMOD works fine starting from Opencart 2.0.1.0 version only, therefore Opencart 2.0.0.0 and beta versions are not capable to work with OCMOD files.
VQMOD is a tool for adding modifications to a store. It is stable, maybe has a few more features for programmers in comparison to OCMOD, but it requires uploading of all mods manually to the server.
Both tools do the job perfectly. They can be even combined together. But I believe that OCMOD serves better because in general it is more friendly to users. You do not have to find and install vqmod, developers should not describe new customers why they need VQMOD and how it works.
Advice: If you decide to install vqmod, go to an official page at https://github.com/vqmod/vqmod/releases
Do not download a custom build of vqmod! I saw many times how these custom builds overwrite directly modification.php file in opencart 2 directory.