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?
WordPress custom post type Single.php?
Wordpress-like configuration for TinyMCE in Drupal
Wordpress: category page not for post's
However, that's not to say it should be ignored.
How to get wordpress plugin “Twitter hash” to show on page
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.
wordpress categories: how to find the last child category for a post
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).
WordPress Rating System
For your second question, they both rate the same as long as the site is structured correctly.
one stylesheet for two wordpress installations
I will state again that people put to much effort into SEO and not into created good content that people want to read.
. In terms of SEO I would Suggest Yoast SEO because it has built in multisite support and makes managing the SEO for every site a breeze.