As far as performance, they should be pretty much the same. The big difference is managability. With a workgroup, every machine has its own access list. That means if you want to create multiple user accounts, you would have to create each account on every machine in the workgroup. With a few computers and just a few users this isnt such a big deal, especially if you dont have to change passwords often.
With a domain, the user accounts are stored on a central server. That way, you only have to create a user account in one place to be able to have that user log onto any machine in the domain.
For example, a large corporation might have 500 users. They might also want the users to change their passwords on the first day of every other month or something like that. If the corp had a workgroup, the admins would be very busy come password change day. With a domain, a user changes their password, and can then log onto any machine.
Hope this helps...PS, for your home network, you would probably want to go with a workgroup.