Stay away from HARD toothbrushes—they will cause permanent abrasive damage to your gums and enamel.
For most people, MEDIUM is also still too hard for our tissues and even though it is very tempting because it 'feels' like it is doing a better job; in fact that 'feeling' is you scrubbing your enamel and gums away for good!
Remember that the plaque that you are brushing away is only as soft as 'wet bread' so there is no need to
scrub hard. With the right technique, you never need anything more than a SOFT toothbrush.