My first JLC was round, a MC Diver GMT. To me it is a fantastic watch and one that I will never get tired of wearing. However it does not fit very well under dress cuffs. My next JLC was a Reverso Duo. It looks beautiful, fits very well under dress cuffs and like its larger brother tells me the time in two time zones. The answer to the question then is both, rectangular for dress and round for sport.
N