I don’t really know if Index could ever change my mind. I really like the world and all, but the reason I like Railgun better is simply because I like Mikoto better as an MC than Kamijo. I think this has to do with the fact that I understand Misaka better than Touya, at least from what I have seen, she seems to be able to use the most logical course of action for something while Kamijo tends to only use his own feelings. That and I can’t stand Kamijo complaining about his luck all the time.
Besides that the only other thing I can think of is I just like the Scientific side better than the Magic one. I like it when series give me an actual way to figure out how much “power” someone has when they are using some magic like ability. This is why I like the Inheritance Series and Umineko. In the Inheritance Series, your power is directly related to how much energy you can weild. In it to be able to use the Magic of the series it takes the equivalent energy of doing the task normally to use the magic. Umineko is somewhat similar in this vein as your power comes from your ability to think and solve mysteries. While Umineko’s one is more vague then the inheritance Series it still is miles better than some.
In the science side, your power relates to how quickly you can calculate what you want to do. This approximately allows us to figure out how powerful every character is compared to another. As far as I know, the Magic side has nothing simmilar to this, plus Kamijo’s hand which I can’t really say anu thing about.
So yeah basically if it makes me like Kamijo more and if the Magic side is more interesting, I will probably like it more, and maybe better than Railgun