I work for a major computer hardware manufacturer (not hard drives), and everything Frank says is right. At the moment the WD Red drives look very interesting, but they are also a brand new product line. I can guarantee you the engineers who built that drive are poring through lots of data so that they can change the manufacturing/assembly/packaging to increase reliability. 4-5 months after a new product line like that is out, it's unlikely any more changes will get made unless a glaring huge is discovered.
Warranty lengths are a marketing/accounting decision, they don't have any relationship to the reliability of a product from any major manufacturer.
I personally use a Seagate 3TB for my parity drive, because it is fast, relatively low power, quiet, and as reliable as any other drive. You could just as successfully use a WD Black or Green depending on availability, price, and priorities for noise/power/speed. I'd recommend waiting a few more months on Reds, if only for the price to come down, as well as for manufacturing and firmware changes to happen.
Given the state of the market, I think it would be a foolish economy to use anything other than an SSD for the cache drive. Spinning disks are still a bit pricey, but SSDs are getting bigger discounts every week. The cache drive is most likely to be operating 24/7 if you're doing any automatic downloading. A good quality 120 GB SSD can be gotten for ~$60 pretty regularly these days.