Go for soft bristles. Soft & rounded bristles are the easiest on both teeth and gums. Many dentists find that brushing teeth too hard is the leading cause of sensitive teeth.
The head of the brush is sized to allow bristles to comfortably reach back molars; since many are too large or wide. It’s worth going with a reputable manufacturer where safety of the plastics & colouring are safe to put in your mouth. As our mouths get larger with age it makes sense to increase the size of the brush head too! The brush handle should also fit comfortably in your hands, & don’t be reluctant to try a powered toothbrush. The average time a toothbrush should be changed is every 3 months. Ask your dentist or hygienist what they would recommend, you might be pleasantly surprised.
If you have further questions feel free to stop by New Westminsters leading family dental clinic, Dr. Lovely & Associates.