Sticking to what are the reported facts, Arneson had the vision of the new model, Gygax wrote it down...
Both are creators, none would have done the game alone. Then if we need to appoint one name only as the "true" creator, this is an opinion and it is subjective according to the info available, his own idea and his mindset...
It is like talking of an artist and an engineer: the artist provides fantasy, ideas and the "emotional" part... The engineer prodides the structure, the plan and its deplyoment...
It is up to everyone decide who (if one only has to be appointed) is the creator...
My opinion is towards Arneson because i am kind of "romantic" and i do see in his part the most valuable contribution.