Well they both use manipulating jutsu, Sasori's puppets and Deidaras clay, but I would have to say Sasori just because his puppets seem like they would last longer than Deidaras clay after he runs out of it.
Yes it must have been a complete coincidence that he said that right when he wanted to trick Kakashi into helping Chiyo and Sakura, so he could take on Naruto alone.
I don't know who's strongest but this can definetly not be used as proof of anything.