You can start Storm instead of Magneto for more than a couple reasons. I don’t think it makes much of a difference who you start. If you’re more of a keep away player, starting Storm would be good. If you like to attack, you have to go with Magneto. You have a 50% or plus chance of catching your opponent in the beginning of the game, and that’s just the beginning of the match.
Storm’s infinite could be ideal in any match, casual or tourny. The only problem is, I don’t think there are that many people out there that can do it on a precise level. Getting them in the infinite isn’t hard at all, and it takes good damage. If you can pull i off, it has it’s usefullness. The only way it would be useless is if that’s all you’re going for and you end up dead. If you can do it well, and end up catching them with a triangle jump lk, feel free to do the infinite.
I would rather have Mags fighting Cable. Magneto should EASILY be able to beat Cable. If Cable does jumping back gun, all you gotta do is dash up, sj. u/f and infinite them. There are so many ways to kill Cable with Magneto, that I advise you to be playful with him. Storm isn’t that far behind though.