If you're optimizing for SEO, you want unique domains for each one. Google gives a special bonus for searches on .com/.net/.org domains. So if you get bananapee

If you're optimizing for SEO, you want unique domains for each one. Google gives a special bonus for searches on .com/.net/.org domains. So if you get bananapeelcompost.com for your new site on that topic, you'll pick up a big SEO boost for people looking for "banana peel compost" on Google. The downside compared to subdomains is of course the cost of the domains (shouldn't be too high if you're not a spammer), and the extra administrative cost. Read http://www.kalzumeus.com/category/seo/ http://www.kalzumeus.com/category/seo/ for more details on the exact topic you're looking into - he has successfully built microsites and written on the topic.