its better to spar medium intensity more often than hard once a week, i tend to see sparring as a training tool, if you are going too hard you will only work on the things you know work and not really improve your game, you need to spend most of your time at an intensity which keeps you honest but which also allows you to try new things without fear of really getting hurt

There is a place for hard full contact sparring but thats a testing ground not a training toll IMO

Now if you are preparing for a fight its different

what ever sparring you do gloves shin guards mouth guard and groin guard just make sense, hard contact sparring head guards should be used but you also need to do some hard sparring without headwear