There are two types of sealants, matte and gloss. I've really only used gloss spray sealant. I've been told that gloss sealants protect better but leave the miniature slightly shiny. One compromise that I've seen is to seal with gloss first, then over spray with a matte sealer.
I've tried brush on varnishes too, but they took more time than it was worth. The fastest method I've found is to set my painted regiment outside and spray them with Krylon Crystal Clear in the same manner that I primed them.