The biggest difference is that the PERC 6 supports RAID levels 6 and 60, the PERC 5 supports 5 and 50. Like I said, the PERC does NOT support non-RAID, so if you leave it unconfigured like you would the SAS 6, then NO disk space will be usable or presented to the OS.  Every disk must be configured into a RAID array, and since the only valid RAID level for a single disk is RAID 0, to use a disk alone/by itself, you would need to configure a single disk as a RAID 0.  Then, in the future, if you chose to convert to RAID 1, it would be a simple matter of running a "reconfigure" from the OpenManage software (done while the system is live and the storage online).
I'm looking into purchasing a Poweredge 2950 G3 server rack to use as a file server, and i currently have a 300GB SAS drive to use as the OS drive, then 2x 2TB drives for the files. So that leaves 3 more empty bays which i would like to use for RAID 0 of the first 2x 2TB, would i be able to put more 2TB drives in? I was told that the BIOS can only see up to 6TB total in the server, is this true and is there a way for i to see more?
Fill all bays with 2TB drives?