In fact you should let people answer 2 of the questions in your poll ^^ it's both due to the fact that he has just 1 eye, and that he has Hashis cells, here is why:
I believe it's because he has just one eye. I also believe there is a lot of difference between the techniques. It's hopefully clear to everyone that Sasuke started to turn blind very farst, and this was obviously due to him overusing Susanoo against the Kages and later in his battle with Danzo. IMO it would appear it happens when you overuse the sharingan to an extent where blood starts to flow from the eye. This goes along with the strength of the technique. Where Kakashis Kamui will seem stronger than Obitos, it would appear Obito is either better at using it since he is an Uchiha, or since he had training from a shinigami...
Anyway this conclusion also makes it obvious that Hashiramas cells will have a huge impact, since it would be easier to use larger amounts of chakra focused to the eye with the healing from hashiramas cells. Remember that his eye is placed in the side of his body that was replaced with an artificial Hashirama body.