quoted

I am in the process of deciding which SATA-II drives should I choose for an
external RAID system. I heard that it was safer to choose drives from
different manufacturers to reduce the chance of near-simultaneous failure
due to model-specific issues.
Is this so?

It's certainly safer.. whether you think it is worth the trouble is
your call.

it would be great to know the numbers.  I WAG that this pretty 
theoretical model-specific failure mode is insignificant compared 
to environment or wear/load-specific ones.  I can imagine the former
was more of a concern in the days of disks on shared buses (SCSI, FC, 
even IDE), when a model-specific response to some protocol oddity
might cause group failures.

nowadays, a SATA disk will probably share power, air, vibration with 
its peers, and in some sense, IO load.  but I suspect there's enough
variance in the latter to make synchronized dying uncommon.  at least 
short-scale synchrony.  vendor would be much less relevant to 
power/air/vibration issues.