hello,
mechanically, they can be quite different, as there are many "systems" of both karate and kung fu.
however, many systems are not thought of in strict mechanical terms. often, they are thought of as philosophical approaches to combat, or life in addition to being thought of as collections of techniques.
with this in mind, kung fu and karate become even more dissimilar.
at the risk of sounding obtuse, there isn't just one answer to your question. this could be the subject of multiple doctoral dissertations.
in the modern day, kung fu styles have become mechanically impractical (though incredibly beautiful) to use in realistic situations. karate on the other hand has become rigid and lacks adaptations to more modern applications, as it is "point score" oriented.
make no mistake though, both (as well as their variants) have a lot to offer. the trick is approaching things properly.
by properly, we mean here: with specific goal, or plan in mind.
many seek to examine multiple systems either "to gather what is useful and discard the rest", or "to compare the many systems".
in either case, there tends to be a "cocktailing" approach to training. if one is going to make a serious investigation into any system, then one should make it a serious investigation. by having a goal, something quantifiable, it becomes easier to recognize ones personal progress, as well as to compare those things the student considers valueable (these values will vary by student).
contrary to popular myth, most instructors are delighted when people from other systems wish to study something new. not only does the student learn something new, the student may bring something fresh to the new school.
a long time ago, this was done through "challenge" method.
"i am here to test your kung fu"
this is now considered arrogant and most instructors (and their students) won't appreciate it.
now then. back to differences.
ranking systems vary considerable.
teaching methods are also quite different.
the physical attributes necessary to move from rank to rank may be dramatically different.
some systems are more sparring (within certain rules constraints) than others.
some rely primarily on knowledge and understanding of prearranges sequences of movements (commonly called by the japanese word "kata").
many utilize different weapons (or even categories of weapons).
if you are interested, try out a couple different schools. most offer free introductory classes, so you will have the opportunity to get a first hand view of how the schools run. you will be able to ask instructors questions and maybe even talk to other students.
good luck.
thanks