The Red drives are better suited for 24/7 operation and are also more suited for use in a RAID array. The green drives are great as well and use less power. I'd go for the green, save some cash and spend it on a game or save if for a Christmas present.
For the minimal price difference I would go with the red. I have the greens in my HP MicroServer, but it's not on 24/7 - mostly one over the weekends only. No issues with the greens so far.
What's the warranty for each? If I recall the greens have 2 years and the reds 3 years - go with the reds