When traveling to Beijing, is it better to join a local tour group or choose a one-day tour after arriving in Beijing?

It depends on your plan to spend money. It will be more expensive to go to Beijing by yourself, and whether you are familiar with Beijing.

It is more convenient to join a local tour group, with food, accommodation and transportation. You don’t have to think about it all yourself. Also, many tour groups nowadays are shopping groups. The prices are very cheap, but they require time to go to shopping spots, so choose by yourself