I just came back from Seoul a week ago. My style is a hybrid of both. Certain tour like visiting the DMZ (border) would require a local tour. Others you can take a taxi or subway and do it free and easy. With an elder, you may want to take more taxi. Anyway, zoom in on your interest, what you want to see, and go from there...

Enjoy your tour... Feel free to ask any questions and I am more than willing to share my experience as I was first timer too with my family.