One big site vs. five smaller sites (the cross-posting problem)

I've been going back and forth over whether to offer one big site or five smaller, topic-specific sites. The difference is all the posts would be in one table if I did one site, with a custom field determining subject. Whereas, with five sites, the posts would live in each site's table.

The issue for me is that of cross-posting. If I have one big posts table, when an article is used in, say, three topic areas, it doesn't have to be reproduced three times and indexed three times. But if I have three smaller sites, then the article becomes a post in all three sites, shows up three separate times, etc.

Any good ways around the cross-post problem while still using multisite?