With the 301, all traces of the site being redirected will be removed from the search engines. You will transfer almost all of it's former pagerank, link juice, etc. to the target site. "Almost" because a tiny amount of that gets lost with the redirect.
If you go the minisite route, both sites are indexed, but then you have to avoid doorway page and duplicate content concerns. You'll also have twice the work, as you'd need to update, promote, monitor both sites.
If it's all the same topic, it's MUCH easier to manage one site than two - unless there's strong justification, go with the redirect.