I love old movies, so when I found 'Singin' In The Rain', immediately loved it and thought the actors were interesting. Soon I started to look for clips of them on YouTube.
Then I discovered this video and was confirmed in my suspicion that Gene Kelly and Donald O'Connor should have made more movies together.
To compare them would be like comparing the merits of apples vs. oranges.
Gene Kelly had a muscular, extremely athletic style of dancing.
Witness this. He dances on roller skates. Yes, you read that right! What's more - Gene makes it look easy.
Donald was graceful - when he wanted to be - light on his feet and athletic also.