In selecting a NTP time server it is important to think about where the device will be located and which timing reference would be most suitable for the applications it is required for. There are benefits and drawbacks to both the radio broadcasts and the GPS timing signals.
Unfortunately not every country transmits a national time and frequency broadcast so if a time server is to be located outside of the US, Germany, UK, France or Japan it may be doubtful that a signal could be received (although many of the these transmissions can be received in neighbouring countries).
Radio signals are also easily susceptible to atmospheric interference and can be blocked by mountains, sky-scrapers or other topography.
GPS on the other hand can be received literally anywhere on the planet. GPS is also a more accurate means of receiving a time code - a typical GPS receiver can provide timing information to within a few nanoseconds of UTC (a nanosecond is a 1 billionth of a second) while national time and frequency transmissions are accurate to only 1 – 20 milliseconds (a millisecond is 1/1000 of a second).
However, one benefit of using a radio broadcast as a timing reference is that it is possible to receive the transmission from inside a building (although basements and rooms with lots of metal furnishings can cause problems), while a GPS antenna needs to be situated on the roof of a building and have a good view of the sky to be able to find and receive the signal broadcast from the satellites (although it is possible to receive a signal through a window depending on the topography outside).