I would always recommend NOT using RAID5 or 6.  I believe that those two raid levels were designed for a time when disk storage was expensive.

by your numbers (6.5TB in raid 5 with 16 drives) My math says you have 500GB drives.  I would highly suggest RAID10.  There are a few ways to handle this but here is my recommendation.

8 raid1 arrays giving 8x500GB pairs.  then raid0 the 8 pairs for 4TB.

Alternatively, A hot spare or 2 is a lifesaver.  use 6 raid1 pairs stacked in raid0 for 3TB and 2 hot spare disks.  I am assuming you have 16 drive trays.  That would give you 14 spindles, 7 of which are active and in a raid0 you will get a very nice improvement in I/O performance. 

You can also break down the raid1 in difference chunks like 4 disk raid1 sets and raid0 the 3 arrays but you will have less active heads (only 3) as all other heads will be writing redundant info.  This is slightly safer because you can tolerate 2 drive failure simultaneously while in the first scenario you can tolerate 1 and then the array will immediately go into rebuild.

As far as filesystem.  I really like EXT3 because it is very very reliable but it is not the fastest thing out there.  It also has very a long FSCK process which is not fun on a multi-TB array.  XFS is a very good performer for backuppc and it is current, stable, and maintained and likely the best choice.  ZFS is platform dependant and hungry for resources BUT is very very fast when tuned properly and can handle your volume management.  It also does raidz/raidz2 faster than many hardware cards and is less impacted by parity calculations than standard raid5/6.  ZFS is a beast of a filesystem and you need to balance the pros vs the cons. 

I suggest XFS.  Pretty good performance and pretty much none of ZFS's negatives(resource hungry, solaris only)

good luck