When multisite is on you have to be careful with your hosting. For example if you want a subdomain installation be sure your webhosting is ready for that because wildcards are necessary (~ subdomain such as *.yourdomain.com).
In your case, everything is focused on users. To me you’d better use buddypress because it’s a great tool to create community websites and it provide a lot of features for your users.
Even if you start with Buddypress you can move to multisite after.