The bag is less relevant than what you fill it with. Most of the ones you find is local authority sports centres are filled with foam or something else that's as useless as a wet kiss in a fight.
The ideal material for it to be still safely usable are rags, fairly densely packed. You can fill it with sand but it will weigh a heck of a lot and you will absolutely need good quality pad mits to work with it (and there's a good chance you'll break your wrists if you're not used to punching something this hard!)
If the bag is filled with too light a material, then it won't provide any kind of resistance and will make your work out pretty uninspiring and it won't be much of a work out. You also won't develop your punching/kicking focus and technique.
Finally, really try to have the bag hanging in completely free space so you can really move around it. Keeping on your toes and moving constantly, like a boxer, is as much a part of the training and the punching and kicking and also teaches you to move.
Daniel San. Best way to avoid punch is to not be there. Hei.