Buy M2 or M4?

Hello, I have saved a long time to buy a m. Now is the time and I would realize my dream. I will test both cars if the weather is better and probably then decide, but wanted to look around before what you see for advantages or disadvantages of the cars. Maybe someone has also driven both cars already? Actually my decision had already been made for the m2, because was only questionable whether N55 or s55. But I have seen that the weight has not changed, I am now undecided. Thanks for your opinions.