Faster drives for parity or not?

[dnLL]

I currently have 4x WD Red drives, 1 acting as parity and the 3 other as data. I'm planning to double up that array with a total of 8x4TB, probably going from 1 to 2 parity drives. I will buy more WD Red drives for my data while the CMR drives are still available (WD40EFRX, not WD40EFAX). At the same time, I was wondering if I would be better putting the parity on 7200rpm drives? Or just "better" drives in general? I was thinking WD Red Pro or WD Gold. 

 

The way I see it, and correct me if I'm wrong... every time I write to a drive, I'm also writing to the parity drive... every time I read from a drive, I'm only reading from that drive. So the parity drives get less read overall (except for parity checks) but more writes than any other single drive. That's until a drive fails, in that case every drive is stressed on every read to calculate the missing bits.