Matt Ahrens has mostly dismissed the idea of using a specific number of disks depending on number of parity disks.
A misunderstanding of this overhead, has caused some people to recommend using “(2^n)+p” disks, where p is the number of parity “disks” (i.e. 2 for RAIDZ-2), and n is an integer. These people would claim that for example, a 9-wide (2^3+1) RAIDZ1 is better than 8-wide or 10-wide. This is not generally true.
The reason the industry has mostly moved to RAID6/Z2 now is that with larger arrays and bigger disks, the chance of an error occurring after a single disk fail, and before rebuild has finished, is reasonably high. Especially is cases where replacement may not happen automatically the instant a disk fails (such as with ZFS on FreeBSD currently).
Personally I wouldn't look at it from a power point of view, the extra disk won't make a massive difference. The question is whether you want to be in position where your array is 'critical' after a disk fail. With RAID-Z1, in the time between a disk failing and you starting & completing the resilver, any error is permanent, requiring you to remove affected files (and any snapshots referencing them). Obviously you have a backup (as you should) so it's not the end of the world, just an annoyance if you have errors, or even another failure, during the resilver.
If you were using all disks in both arrays, it would be a question of do you want the extra redundancy more, or the extra space. As you are only going to use 3 disks for the RAID-Z1, giving your 2x2 TB of usable space in both cases, I think I would just go for RAID-Z2.
The only other concern that could affect you, is that with all disks being exactly the same and purchased together, you could argue there's a slightly higher chance that more than one may fail at around the same time.