We have some MS SQL servers that are setup with different instances on the same server to separate applciation DB's as well as some servers that are setup with all DB's on the same instance, just separated with security settings.
When is it advisable to create a new instance for SQL server and install your DB's in that instance as opposed to just creating a new DB on the same instance and putting security around the database itself? Is there more to the decision that just a security aspect?