[M2d]

Matter of taste. Gene Kelly was more along the vaudville style solo dancing while Fred Astaire was better known for his ballroom dancing. Gene Kelly was also known for very inventive uses of props and scenery in his dances. imo, Astaire was more versatile, but Gene Kelly was more fun to watch.
Check out Gene Kelly's roller skate scene in "It's Always Fair Weather" for some mind blowing tapping.
other scenes of note (not the famous ones like the dancing with shadows one):
Fred: Astaire/Rodgers dance scene in the dance class studio in "Swing Time"
Astaire solo dance scene on the driving range

Gene: the aforementioned roller skate scene in "It's always Fair Weather"
Kelly, Kidd and Dailey's synchornized softshoe to The Time for Parting in Fair Weather
with Judy Garland in "For Me and My Gal" (movie and song of the same name)
almost all of "An American in Paris"