Love both places but they are like light and day.

Singapore, clean, like spotless clean, modern, safe.........but not so tantalizing, kind of sterile.

Bangkok is not clean, no where near as modern, not nearly as safe..........but it is very interesting......lots of culture.....not sterile.

If you have time I would stay in Bangkok 3-5 days and either fly or train to Singapore for 2-3 days. Then fly home.