Our workgroup has under 20 computers (Windows, Mac, and Linux), so it's small like yours, so our requirements should be similar to yours.
If you aren't looking for a free solution, then I would recommend Retrospect. Our final setup is like this:
- Retrospect server is running on Mac OS X 10.3 Server, DP G5
- We use firewire drives for offsite backup (once a week)
- We have nightly backups to the server of all clients, using incremental backup
- Our network is 100 Mb, with Airport Extreme on some of the clients
We explored Bru, Netvault, and Retrospect as options.
Netvault was running on our Redhat server. It's interface was rather buggy at the time, and is not very intuitive. Netvault is, after one gets over the learning curve, quite powerful though. It supports backup to tape, file, whatever. We were interested in the backup to file option, as we wanted to use firewire drives for off site storage. Netvault has finally announced official support for Mac OS X, and has server software for Mac OS X, it appears. I have not tried it, since we have invested in Retrospect. My rating (3/5)
Bru was the very first one we played with, again, at the time it did not have a server version for Mac OS X. It was very long ago, but, I liked it even less than Netvault. My rating (2.5/5) it's not bad, but for our special requirements (small workgroup) it didn't work well.
We finally settled on Retrospect. It's the solution that we trialed, and have now purchased and deployed. It works very well, backing up our windows and mac workstations. The only feature I wish was better implemented, is controlling exactly what folder one wants to backup on the client (we want to backup only important data, not preferences and all of that stuff, we finally settled on backing up the users folder). My rating (4.5/5).