It's not a fair comparison in 616. In the sixties you had real character Gwen vs. comic relief MJ and, after that, you have real character MJ vs romanticized memory non-person Gwen. You can't even compare sixties Gwen to later MJ, since there's so much difference between the two periods in the attention supporting characters were given in terms of detail and backstory. It probably comes down to what period one considers to be the definitive Spider-Man. I think of it as the post-Ditko Lee-written issues, so I'd go with Gwen, someone who considered it to be the 80s would go with MJ.