Cotton is a plant fibre, made from cellulose, whereas merino is an animal fibre made from protein. Because of its construction cotton is softer against your skin, especially if it’s high quality cotton. Merino wool, though softer than most other wools, isn’t quite as soft as cotton. It is on the other hand much better at regulating your body’s temperature. Incredibly, a jumper made of merino wool will be much better at keeping you warm and much better at keeping you cool than one made of cotton.
Both merino and cotton are hard wearing but cotton is slightly easier to wash. Because of the fabric construction, a cotton jumper is less likely to catch or lose its shape which makes it easier to care for.