Right, although you can always implement both and create both as arguments, for instance going to http://example.com/userstores/0/123 could be setup to return nodes from any group associated with user 123. You just need to be familiar with Views (and their Arguments) in order to get that to work, but it could be done fairly simply.

Your first step is to look at the og_ghp_ron View, which is the default Organic Groups View, and see how that works.

Your best scenario is to force (or encourage) users to create a Group, as that gives them the option to create their own store front with a news feed and everything, such as we've done at http://rifftrax.com/iriffs. (Click on any of the "iriff by" fields to see a homepage.) We haven't done anything as fancy as the "userstores/0/123" argument functionality yet, but that isn't a bad idea now that I've mentioned it

If you still need help let me know, but familiarize yourself with the OG view and Views arguments and you'll be better off in the long run!