What Dan said.
If you're not familiar with the difference, a redirect will take the user to your sub-domain. If you mirror the sub, the browser will stay an yourdomain.com.
If you care about Google PR and already have a bunch of inbound links to the domain, you might be better off mirroring it, otherwise a redirect (301) would pass the PR to your sub.
You could also just move everything to your domain folder, then redirect the sub to your domain -- if it's something that already has incoming links.
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