The ones that give you other ways to subscribe besides email, is how I differentiate. I am not interested in getting articles in my email, it's for correspondence only.
I don't promote it much because it has some disadvantages from my end, but I see the value in RSS. Hell, I rely on them to make each issue.
I use my inbox as a todo list, and something like Feedly for my article consumption. It's hard to mix the two successfully.