I had a chance to try out the new Google Groups Beta the past couple days, and I must say it is a great upgrade.
The interface is more user-friendly, users can now upload files, and contrary to Yahoo!'s Group offering, users have the ability to create custom pages using tools that were originally only available using Google Page Creator.
The one HUGE thing that is missing, is the ability to easily integrate a Google Calendar into the Google Group itself. I tried embedding the HTML of a calendar for one of my groups, but alas, it doesn't seem to work. Once they add this feature (which would be a logical move), a Google Group would be far superior to a Yahoo! Group.