Shared vs dedicated boils down to a few primary things:
1) Your skill level. Share hosting includes all that back-end monitoring and configuring. Where as on dedicated servers you are the administrator and control almost every aspect of the server's configuration and software lineup (and diagnostic and problem resolution). With that a certain level of experience and knowledge is recommended and required.
2) Cost. Shared is less expensive; dedicated more.
3) Speed and security. Shared is shared -- so the opportunity for resource issues and compromises are more common. With dedicated -- it's dedicated -- so the speed is all about you and how you config your site. Dedicated is more also secure.
Judith "Do one thing every day that scares you." ~ Eleanor Roosevelt