Hey there, I am thinking of using wordpress multisite for a network of 30 blogs.
The questions is: every blog has its own directory, files etc.
With multisite and domain mapping, everything will become virtul, is this good for seo?
Second question: which do u think ranks better, a simple html seo site or blog?
If you have good content SEO doesn't matter. However, that's not to say it should be ignored. You can use WordPress MU with either subdomins like myblog.example.com or folders like example.com/myblog I prefer subdomains but both are fine for SEO. A third option (and sounds like what you would like to do) is map each blog to a domain such as myblog.com and myotherblog.com which is fine too. I think the main thing is if you use domain mapping each site is being seen as a different site instead of an extension of the original site (I could be wrong here)
For your second question, they both rate the same as long as the site is structured correctly. I will state again that people put to much effort into SEO and not into created good content that people want to read.
Using multiple site directory is not good option If you are having multiple directory option that is not good for SEO. Using multiple author that in one blog with good aritcles is really good options for you...Post title and wordpress plugin called seo ultimate will help you so much... I you want more information about SEO and wordpress read the wordpressapi.com blog.