Site Defs Vs Site templates

Site Definitions


Files are on disk, better performance.
Highly customizable and extensible (XML and .NET code is much more flexible than UI)
Can provision multiple webs
Complex to create
Requires admin access to server for deploying.
Modifying the site definition after provisioning at least one site from it is not supported and might break existing sites.

Site Templates


Files are in database, less efficient.
Not easily extensible (You are limited by what UI offers)
Can only provision one web
Easier to create
Installable from a Web UI by site owners.
Custom templates can be modified without affecting existing sites that have been created from the templates.