It's a very very close call.
Dorian trained brutally hard but only over short bursts. Arnold's intensity was severe and was usually done twice a day for workouts lasting several hours in total.
But then Arnold didn't train legs or back very hard, calves yes, but legs no. That means that he was neglecting nearly half his body. Dorian never had to train calves very hard at all - it was mainly genetics but he still trained them hard, heavy, and regular.
You've got to look at it from a whole body perspective. Dorian put 100% into every bodypart - Arnold trained hard and was focused in every area of his life whereas Dorian was all about the bodybuilding and that alone.
When it comes to training in the gym, Arnold trained longer - but I don't see how it could have been harder as no-one could sustain Dorian's intensity over that length of time that Arnold was training.
So to conclude, in terms of pure training intensity alone, I would give it to Dorian.