The Red is anytime better than the Green. For NAS usage, I'll avoid it like a plague.
Actually, I'm also in a dilemma. Initially looking at 3TB drives but I heard that there will be some incompatibility problems with the Microserver. So now looking at 2TB variants. For the same price, I can either get the Hitachi 2TB or the WD Red. I believe warranty wise both are the same. The only thing that is pulling me back is the Red is 5400rpm while the Hitachi is 7200rpm.
Not required. The Red is already built for NAS environment albeit only for home and small office/home office usage.
It used to be Hitachi but at the moment there's only Seagate and WD with very little involvement from Toshiba to carry on what's left off Hitachi. If you want Seagate's reliable line of drives, look at their Constellation series. For WD, it's their Black series or their RE4 series. Both are expensive but reliable.
But then again if you're looking at storing and keeping the hardisk locked away after one time use, any decent brand would be fine.
Hardisk would be fine for long term storage if you take into consideration the price per GB and the speed. Tape is expensive while still slow.
If the data is very important, make an exact copy of your backup in another hardisk and keep it somewhere else. From time to time, take the hardisk out and check the files at random to see if the hardisk is ok.