I like the OT version of turning to the dark side. The lines were a little more blurred. In fact, Darth Vader was always supposed to be in this grey area of the dark side since Luke could see good in him and so did the audience since Vader never seemed to really wanted to kill his son. By watching Luke and the Emperor interact, we sort of learn by killing out of anger, you would be maybe turning to the dark side. Nothing is for sure because it seems alot of factors come into play such as the strength of character.

the PT makes turning to the dark side much more typical and obvious. do evil and act like a whiny bitch and obviously, thats dark side. they made anakin turn into pure evil which kills the idea that there may still be some good in him. yea he showed some concern for Padme but it felt empty (as did that whole relationship in the first place). turning to the dark side here was definitely believable, but boring to watch, predictable and melodramatic.

by the way, i think both OT and PT needed to say more about the benefits of staying in the light side... or the disadvantages of the dark side (besides acting like a douche)