At some moment I would say Groups too, because now-a-days facebook has disabled the option of sending invites for the fan pages for everyone, except the admin. So its pretty difficult for an admin to get more fans on its pages.
I closed my group page, and started a FB Fanpage for my services, and it worked out much better....just don't forget to update it. Give away something and always very useful information.
I like groups, you can get members and send everybody messages, I didn't find the same thing for fan pages
Facebook pages are easier on the eyes and have plenty of options which are missing on groups (even the new groups). Also, after today's additions to pages, its a must get!