The ZS15 is the USA model.
The TZ25 is the Europe model
They are the same except the European model has a limitation on how long you can do a continuous video shot. This is due to a European tariff on video cameras.
The tariff is on video cameras, but to keep a digital camera from being classified as a video camera (and taxed higher) Panasonic chose to restrict the video recording time. Therefore it gets a different model number in Europe.