They weren't the same planets. According to wikipedia, Prometheus landed on LV-223, while Alien/Aliens takes place on LV-426. Remember, Prometheus is not a direct prequel to Alien/Aliens. Instead it's story takes place in the same universe as Alien/Aliens. Ridley Scott has said that not all was explained in Prometheus and he wanted to do a sequel to it, so perhaps the sequel(s) will tie directly into alien/aliens at some point.
Good deal from Amazon...I'll probably pick up the $20 version. As disjointed as the movie was, I still thought it was visually spectacular(though disappointing). I wanted to wait for a directors cut, but who knows when that'll come out.

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