Dubai vs Singapore Stopover
In Dubai you are stuck in the airport, while Singapore lets you take a visa (free of charge) provided you show the boarding pass of the connection flight (and are an American green card holder). Dubai authorities on the other hand expect you to stay couped up in the airport!
As a side note, while Dubai is struggling with traffic congestions despite its six-lane highways, Singapore traffic is relatively a breeze. The underground tube railway in Singapore is widely used and very efficient while the one in Dubai is still under construction.