The climate in Iceland is much milder than it should be considering it‘s location at the south of the Arctic Circle, so you can expect good weather when visiting; however, be prepared for possible bad weather occasionally.  Summers are calm and mild, and sunglasses are a must.
Greenland is far colder as the ice is ever present, and winters reflect this by being pretty severe.  Snow is more heavy around coastal areas, and in the North of the country, the temperature only rises above freezing briefly in the summer.