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SATA disk use main CPU to process data transfer, so they slow down the PC. With heavy disk access the slowdown may be signifficant. Besides, top speed for SATA drive is 10000RPM (only one model in varoius capacities), while top SCSI speed is 15000RPMS with random access in sub 3ms. (sub 5ms for fastest SATA). Especially with software basd (and some cheaper hardware controlled) RAID 5 configurations SATA can be painfuly slow.
SCSI drives on have also their own own processor, so data transfer doesn't slow down the main procesor.
As for the redundancy, for the last 5 years I've been quite happy with RAID 1 on my two home servers. RAID 5 is a little more economical and can be faster, but the speed issue depends on the actual applicaion. it is not always rue that RAID 5 is faster than RAID 1. For instance for database applications RAID 1 may be a tad faster if majority of operations are writes rather than reads.
As far as speed goes, of course RAID 0 is the fastest, but using RAID 0 is ony for those who are not very sentimental about the data they store on their drives. One glitch on either drive means and you kiss your data good bye.
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