Since you asked a Q like that, I assume you haven't spent any time in Asia. Obviously, it depends upon so many factors you did not provide. In general, Singapore is quite westernized unless you seek out the Chinatown, Little India, and Arab Street areas. Why not just go to San Francisco if you take a vacation and only go to Singapore? You can and probably should go to multiple places such as in Malaysia on a trip to Singapore. You can also fly to a place like Bali quite easily from Singapore.

If you go to NRT and have the time, I would suggest that you purchase a rail pass and take the trains, including the bullet trains, and see much more of Japan rather than just Tokyo. Sure, spend two or three days in Tokyo and see the sights as well as just walking around, but then go to Kyoto, Nara, Hiroshima, Miyajima, and others. Those other areas are the real Japan, and much less expensive too. Have fun.