Multi sub-domain database configuration best practice.

What is the best practice to use a backend database for configuring multiple sub-domains with a single drupal installation.

Is it better to share one database and use db_prefix to differentiate/share the tables across sub domains or use a different database for each sub domain ?

Few of my constraints are

** Share user registration information across sub-domains
** Seperate blocks, content and taxonomy terms for each sub-domain.

If using a single database with db_prefix, will it be able to handle all the transcations related to multiple subdomains in a long run, thinking that main site may develop in to some where around 25 sub domains.