Basically, the green drives are entry level, with "green features" that allow for power savings, and should be considered the entry level drive from WD. The blue drives have cache from 2MB to 16MB and a 2 year warranty, and are the mid range drive. The black drives have up to 64MB of cache and a 5 year warranty, and are designed for performance/reliability. The red drives are standard 64MB of cache, 3 year warranty and are designed for NAS/RAID environments.
In my primary PC's - I use the WD Black drives (primary gaming, wife's computer, HTPC). For my non-essential PC's - I use the WD Blue drives (guest computer, test computer). The black drives will work well in almost any configuration, from single drive to RAID.
Seagate makes good drives - but I will choose Western Digital over any manufacturer of drives....just my opinion...