From what I've heard second hand the Onstream drives are excellent for workstation or small network backups. I'd recommend staying away from 4mm DDS2 DAT drives, because they tend to develop head alignment problems that cause old tapes to be unreadable. DLT drives are expensive, but they're very fast and reliable and you can find multi DLT changers for backing up huge amounts of data. You probably wouldn't be happy with the Travan drives. I have the Aiwa USB Travan drive at home, and it's good enough for basic backups, but it's loud, slow and only fits 8 gig on a tape (compressed). Of these, I'd recommend the Onstream for your setup since it offers a lot of the benefits of DLT at a significantly lower cost.
Tony