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I completely understand the not mixing franchises thing. But the age thing makes no sense. Ben totally looks younger than Rob Pattinson. I also think Ben is better looking -- Rob just looks freaky as Edward (although I did like him as Cedric in Harry Potter 4).
As far as acting goes? I have no idea. Neither one of the young men have done hardly any other films besides Twilight and Narnia. I've seen Ben in "Easy Virtue" and Rob Pattinson in "Remember Me" and just based on those, I'd have to say that Rob is the better actor. However I really do not like the way he plays Edward (laughably at times), still, it's impossible to know whether that is Rob's fault, or direction/writing. Would Ben have been a better Edward? I definitely would have enjoyed him more but that's only personal opinion.
Leaving aside the shocking idea of "Prince Caspian as Edward Cullen!" -- who do you think would have really been the best actor for the part?