WD's Green Power drives run at 5400RPM, period.
No lottery, no variable speeds, etc.
You'll never see WD state that in their HDD propaganda, because they don't want to scare off semi-knowledgeable buyers, with buying "old technology" 5400RPM drives. While it may spin slower, the WD Green Power's disk density is quite good, so sustained read/write performance isn't lacking (as much) when compared to current 7200RPM drives. Latency will always be a bit slower though...
The only reason to get the WD Green Power drives is for a cheaper large capacity (>1TB) storage drive or for something that uses less power/runs cooler.
If you're purely looking at performance, stick to a defined, current, high density (>320GB per) platter 7200RPM drive...