I have a black belt in Tang Soo Do, and currently and quickly I'm closing in on one in TKD (ATA branch), via belt skipping every testing. IMO, they are nearly identical in basic, fundamental techniques, though the forms and self-defense moves are significantly different, but them I'm taking Songahm which has had its forms completely revised in the late 80's, to add more modern day maneuvers, and add lots more kicking to the forms.
FYI, Tang Soo Do was the style Chuck Norris was trained in and taught. I consider them generally equal in technique, attribute requirements, and effectiveness. Therefore, if you had a choice between the two styles, I'd pick the better instructor -- but then, that's almost always the way to pick a school/style.