@GUIn00b said in Practical RAID Decision Making:
One particular situation where I'd find it not quite so straight-forward to go for RAID-10 vs RAID-6 is in a 4-drive setup. Some things to consider would be the performance capabilities of all devices at play (drives, HBA, CPU, etc.) as well as the performance demands of the users/services that need frequent access to said storage. For me, if I/O demand isn't real high for services (and probably using flash, not spindles) I'd be willing to go with RAID-6. Though both RAID levels can sustain 2 drive failures, the caveat with RAID-10 is as long as it's not the same member from each mirrored set. With RAID-6, ANY 2 drives could fail and still be operational and recoverable. I guess it would have to be a very specific concern to opt for the parity overhead in favor of the "added protection" over a statistically very rare potential failure scenario of 4-drive RAID-10.
OK nvm. RAID-10 + backups. 
edit
https://www.arcserve.com/blog/practical-raid-decision-making
RAID 10 for four-disk array
Likewise, with a four drive array the only real choice to consider is RAID 10. There is no need for further evaluation. Simply select RAID 10 and continue.
Well, SHUT MA MOUTH! 
Consider life expectancy of a RAID 6 over a RAID 10 as well. It's SIGNIFICANTLY more write amplification due to additional parity over RAID 5. RAID 10 would be the best option for having the best possible usage/life expectancy for your drives.
During rebuilds, RAID 6 is the devil. That could be enough writing to make more go belly-up. Then you're toast.
I'd argue that over the lifespan of a server, RAID 10 would likely save more money/resources and headache (and data), making the initial higher cost of capacity worth it. Not only that, but there's other benefits as you mentioned such as speed, iops, etc.