I have moved for both reasons and each are equally valid. Have you spoken to your line manager about opportunities within your current school? or even your head teacher? They may well offer you opportunities that are in the pipeline, as it is a win/win, you get the developemnt and they get to keep a valued member of staff It is so difficult to know if you are moving into supportive school with a good ethos towards staff and pupils.
If you are happy and still enjoying your job - why move? The grass is not always greener ... I have moved jobs twice for professional development and once to try something different (and in all those cases for more money). My first school had a very, very stable staff and the only way forward for professional development was to move on or wait for someone to reach retirement. Many teachers held their posts for 20+ years. It was a very happy school.