We use A, because the other one didn't make sense to us. Note that a "project" with regard to SVN is not necessarily a single project, but may be several projects that belong together (i.e. what you would put into a Solution in Visual Studio). This way, you have anything related grouped together. All branches, tags and the trunk of a specific project. Makes perfect sense to me.
Grouping by branch/tag instead does not make sense to me, because the branches of different projects have nothing in common, except that they're all branches.
But in the end, people use both ways. Do what you like, but when you decided, try to stay with it :)
As an addition: We have separate repositories per customer, i.e. all projects for a customer are in the same repository. This way you can e.g. make backups of a single customer at once, or give the source code of anything the customer owns to him without fighting with SVN.
