Access and travel logistics
Spain is generally easier to reach from a wider range of international starting points. Frequent direct routes serve the Costa del Sol, Barcelona, Mallorca, Alicante, and Valencia from the UK, North America, Canada, the Middle East, and across Europe. Greece has strong connections to major European cities and, increasingly, to the US and Middle East, but fewer direct routes from North America and more complex island access.
If you are based outside Europe, access should be a central consideration. A second home in a location that requires two flights and an overnight stop will be used less often than one that is three direct hours away.