multisite or multi install?
Your choice of site architecture (e.g. multisite or multiple drupal installations) doesn't necessarily play into performance. Meaning any site can be made performant using the right mix of modules (memcache, Varnish on the front end, etc.);

With that in mind, look at the following criteria when selecting multisite over multi-installation instead:


Will most of the modules on the sites be shared with one another?
Related, will the modules always be in sync via version? i.e. Context 3 vs. Context 2.
Will the versions need to be in sync? i.e. If you upgrade the main site, will the blog site also need to be upgraded?


If the answer to any of these questions is yes, use a multisite. If there is concern about them, use two document roots with one site a piece. Sharing content can easily be done through any of the following projects so multisite or multi-installation doesn't matter:


http://drupal.org/project/feeds
http://drupal.org/project/migrate
http://drupal.org/project/services