Between the two celebrations there is a similarity that is: they symbolize the step of becoming a girl to a woman. However, there are notable differences between the parties of 15 years and the sweet 16.
Sweet 16 is a tradition celebrated in the United States and Canada, while a quinceañera is a tradition celebrated in most Latin American countries. In general, quinceañeras are more formal, with traditional dances such as the waltz. This party has a more marked protocol to fulfill as opposed to a sweet party 16.
Sweet 16 parties can be much less formal than one of fifteen years if the party wishes it, you can choose the theme you prefer, it can be a great party or something simpler, since there is no parameter to fulfill.
In conclusion, both parties have the same meaning, it is up to each family to decide which of the two to celebrate, although in some cases they celebrate both parties.