Karate is Japanese. Kung Fu is Chinese.
The forms of Karate tend to be simpler and more to the point. Kung Fu forms are more complex. So it would take a person longer to assimilate Kung Fu, given equal work and talent.
Karate has a few distinct styles, while Kung Fu is really a great number of martial arts with certain family characteristics. So if you were interested in Kung Fu, you would have much more things to choose from in terms of style and more variety between those styles.
As a general rule, Karate relies on powerful strikes, while Kung Fu puts more emphasis on speed and agility.