I like the way the HP does file redundancy. They just copy the file twice, making sure the copies are on different disks. This has a number of advantages.
Each disk is a windows formatted disk, so if the server dies, you can plug the disk into any PC. I am not sure about file recovery with RAID, is it standardized, so you can take your RAID disks and plug them into another machine?
You can mix and match disks, they can be any size.
You can decide which files are duplicated, and which are only stored once, so you can trade off relaibility and storage capacity.
I am a PC guy, so I cannnot comment on how nice they play with MACs, there are a couple of SmartServer forums, I am sure they will have some answers.