I want at least some flex in the mallets to absorb some of the shock of the attack on the bar, otherwise I begin to feel the results in my arms and elbows. But, I don't want the shafts too whippy, either. It's a delicate choice. It the shaft is too thick, it feels uncomfortable and clumsey, and if it's too thin, there isn't enough stiffness to play right. It has to be just right.
Nylon never wears out, so it's the best wrapping for the mallets.
But, it's definitely the best material. My guide for the right hardness of the mallet is that I want to be able to play across the dynamic range from very soft to very loud with a minimum of change in the tone of the instrument. A mallet that is too hard sounds good at soft levels, but becomes clanky and unpleasant at louder volumes. A mallet that is too soft just doesn't have the bite to cut through at higher volumes. Again, it has to be just right.