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I have a site www.product.com and another site www.productblogg.com. The content is different on the 2 sites but they "talk" about the same product. and... the www.productblogg.com use a multi-blog-system so there are about 20 blogs under that domain that all link back to www.product.com
Is it better to have www.product.com/blog istead of having 2 domains?
A second domain isn't likely going to help you, do you expect it to show up in search results above or below the results of your main domain? If it's not linked to and content isn't good quality it'll have no use. Having your blog on your main domain you have more pages to potentially become site links. Linking from your blog domain to your main domain when it's not authoritive will have little value. You then also have visitors on your blog but not your main domain where you probably need them to convert into sales / contacts.
Having them click between two domains is most likely going to drop your conversion rate if it takes longer for them to get to a contact page or product page. I don't see companies with separate domains for blogs very often. I would be deciding on either a sub-domain blog or folder blog. Not for SEO purposes as that doesn't matter just for ascetics and what you prefer mostly over anything else blog.example.com or example.com/blog. Since you haven't yet setup the blog and you're still deciding on a domain i'd suggest you spend more time on building out your primary domain then worrying about where to put your blog.
Whilst I agree that having everything under one domain makes things simpler, one alternative is to use a subdomain, like blog.product.com. Then have productblog.com permanently redirect (301 response) to the subdomain. If you already have pages on the productblog.com domain, use a path mapping redirect, e.g. Redirect ^(.*)$ http://blog.product.com$1 [R=301]
You should make sure that your primary site uses no www subdomain for its canonnical URLs, so that Google will say "see more results from product.com" (which will include blog.product.com) as opposed to "see more results from www.product.com" which would then exclude the blog pages.
Another suggestion if you use two domains or a subdomain is to add rel="me" attributes on both linking to the other. This shows that you are in control of both domains and that they represent the real-world entity.
