Thank you all for your input.
I actually purposely left out a few of the details because I didn't want to contaminate or influence the opinions of the people who have been providing them.
So, I'm going to fill in the back story now.
My current RAID array is 10 * 3 TB drives on RAID5. They're Hitachi 3 TB drives that I bought a couple of months before the floods in Thailand, not knowing that there was going to be a flood, but because they worked out to be like $114 each per drive, so it was a really good deal. Yayyy eBay.
My original intent was to have the first 10 drives (out of a box of 20) to be deployed in my current server and then the second group of 10 to be deployed as the backup; BUT now you might be asking "so why did you ask?" well...I want to make sure that that was still the better way to go and that LTO and BD-DL or BD-XL hasn't replaced it in terms of cost and viability and reliability.
I haven't built the second backup server yet because I'm still debating as to whether I want to keep it as NTFS or whether I want to migrate back over to ZFS as the backup server. (Aside from one or two other experimental file systems, those are the only ones that will support a single volume that size.)
I actually already have the drives, I'm just waiting to get the enclosure and the rest of the server hardware (motherboard, CPU, PSU) etc. before I can launch and deploy the backup server, but I just wanted to make sure that the initial plan still held its own when tested by other people.
And given the feedback, it seems like that hard drives is the way to go, although trying to expand the capacity of either systems would be a little harder, unless I just get bigger drives, but the same number of them.
Thank you all for your feedback/input.