It's recommended to choose both the same model for RAID purpose, and AFAIK WD Green is not good for RAID, as it'll reduce the life cycle of the read/write head sit.
If you really want speed from WD, get the Black Edition instead, but most of us here would surely recommend Samsung F3 1TB, get 2 or the 500GB version (not sure if it's F3 or F2) and RAID0 them, cheaper, and way faster.

You won't see any different in standard and RAID edition Hard disk, in fact they're pretty much the same, the very high MTBF on the RAID edition means they have higher chance of longer life time, does not mean it'll not fail.
If you're not running huge server with lots of RAID drive, then by all means just get the standard drive, they can also put into RAID if you like to do so. (Software RAID)
For more information of function of RAID, google "wiki RAID".

This post might help you: http://forum.synology.com/enu/viewtopic.php?f=26&t=19674
Hopefully it solve your problem.

I'll say this first as I'm in no offence of WD Black but the speed compared to Samsung is slower and the price way higher (not sure what so good about the premium).
I'm using one so I know about this, the heat is an issue, way too high in my prespective, if you really want cool harddisk, take Samsung, for the 1TB F3 version, it's faster, cheaper and way cooler than WD Black.
But, if you're royal fan of WD, then by all means, get a better fan to cool it down fast.

samsung more eco frenly than the rest haha