Fact 1: My server will always have exactly 5 drives installed because that's as many as the case can hold.
Fact 2: There will be an off-site backup solution in place.
Did I say hello, btw? Sorry. Hi! 
Now: What's the Best™ configuration for 5 drives? They're all 4TB WD Reds connected directly to the mainboard.
a) All 5 in Raid6/RaidZ2
b) 4 drives in Raid6/RaidZ2 and the 5:th as a backup. (a real back-up, not a hot spare)
c) Same as b but with Raid10 instead.
d) <insert Best™ idea here>
I don't care which configuration makes it easier to add more disks (see Fact 1).
I'm fully aware that Raid isn't a substitute for backup (see Fact 2).
