| Today I bought an Ultra 1 with cgsix.
| And I am finding SBUS fast wide scsi adapter to use 12 external hard disks.
| Do you know which one is better ? (ncr or qlogic)

It depends on what you want.  The isp HBAs have a microcontroller that do
practially all the work.  fas is managed mostly by the main CPU.  The CPU is
many times faster than the microcontroller in the isp.  So fas should have
lower latencies, but isp should have higher throughput.

| And should I get a differential type ?

These are not LVD.  A differential controller only works with differential
disks.  You cannot mix the two.

Eduardo