In another OT thread 'silverwheel' explained perfectly why it should be Sebastian Shaw, better than I ever could:

"It ruined one of the most powerful lessons of the series. The ghost of Old Anakin symbolized that while he could not undo the many terrible things he had done, he could still make amends and seek forgiveness. Old Anakin symbolized a wasted lifetime. Hayden's ghost basically gives the character a free pass. Plus, Old Anakin fits with what we had just seen in the movie - a tired old man trapped in an iron lung, desperately wanting to get out. Now, in death, he has his release. The notion that his ghost can just "go back" makes no sense, literally or metaphorically."