If you choice is between taking a striking art or doing extra classes in another dojo, i would take the striking art.
There is enough difference between striking arts and aikido that the chance of getting them mixed up or taking 'bad' habits from one to another is minimal. i have never accidentally kicked someone when practicing irimi nage. and other things will actually help you art learning how to punch is a good thing and many aikido dojo don't teach this well.
you are far more likely to create confusion but training under 2 sensei in the same art, there is a good chanc they will have difference emphisis and move in a different way but close enough to mess you up.
as for choosing between karate and TKD, have a look at the classes and see which you prefer, no if you don;t enjoy the class you won;t get anything out of it. if it is ITF TKD then the classes may be quite similar as the root of ITF TKD lies at least in part in shotokan. i also agree with Seiser Sensei that karate will have similar terms etc. so may be easier on your mind