For free, Google Drive gives you 15 GB while OneDrive is changing in August to only 5 GB (they recently sent out an e-mail about this). So, that right there might effect your decision. I have been using Google Drive a lot longer, only signing up for OneDrive after downloading the Word apps for my iPad and Android phones (previous phone and now this one) within the last six months (used Polaris for documents before that and it became buggy), and also use Dropbox because of certain apps (especially iPad apps) that work only with it and not Google Drive.
I don't really use any of them for backups, though some of the files are there partly for safe keeping. I use Google Drive for all sorts of file types (including PDF, photo, and video) that I want access to at all times on various devices, including important documents. I use OneDrive just for the Word mobile app files for my tablets and phones (previous and current phone). I use Dropbox for the apps that use it, for sharing files with others sometimes (which I have also done with Google Drive), and as a quick way to share personal photos between my iPad and Android devices. I prefer Google Drive, so have the most stuff on there and everything organized in folders on it. I need to do some cleanup on my Dropbox of photos I don't need on there anymore.