Oooh! This is a tough question to answer.
One could make the obvious generalisations about the differences between Karate and Tae Kwon Do.
I suggest you go and see/try both classes and decide on the basis of the quality of the instructor and the class.
You should also consider which best complements your main art as it sounds as though your not really interested in learning either fully.
Do you have friends skilled in any punch/kicking arts that can help you with this it may be more useful to book a squash court now and again and get a freind to teach you the basics of punching & kicking.