I've just done this. A few thoughts / comments
I Used an HP SCSI DAT, firstly because I scored the drive brand new real cheap and had a few reasonable SCSI Cards on hand. Secondly because I like SCSI for tape drives - good throughput without pulling the server down too much. Thirdly because it had tapeware bundled which was kosha and works real nice (once one has grasped the philosophy's behind it) and lastly because DAT media is real cheap to buy.
The Tapeware interface is a bit ugly but it works and once setup, should require too much attention. I worked out a way of using a windows box and the nice GUI interface to control the software in the SME Server.
I hate DLT units - use them at work and they are frequently coming unspooled. Easy to sort, just fiddly and time consuming and the media is a bit expensive to keep munting.
The final choice will come down to budget, capacity and personal expectation.
Finally - must acknowledge the list for the final snippets of help to get it all going, in particular retro fitting a SCSI card