Go with option A. If you back-up your subversion repository by backing up your subversion server, there is no need to back up your working copies*. When changes are committed into SVN, they get transferred to the SVN server, so by backing up just the server you are in effect backing up the working copies barring any uncommitted changes.
In addition, Subversion is very optimised for network transfer when communicating with the SVN server, but is quite heavy on disk usage/access for the working copies. By putting the working copies on a remote file share, you are getting the worst of both scenarios.
* any changes not checked into to SVN will not be backed up, but I would advise that you encourage people to commit regularly. If this is not easy due to partially-complete functionality, take a look at branching in SVN. You could give each developer their own branch to develop on, and only merge their changes back into the main trunk once changes are complete.