As said, 10 year old DX9 code still works on DX9 today. With OpenGL you can build with new features on old things, but I think it's mostly a bad thing - it's why OpenGL is a terrible mess nowadays if you don't go for the strict core versions that don't have any deprecated features.
In the end there's not that much between them; I'd choose the one that feels more comfortable to program with - try a bit of both and then choose.
are you sure about that? because I want to read jim adam's advanced animations in directx. it uses directx9 but reviews says the code is incompatible with the current directx9.
Its these kind of things that make me angry about updates to APIs. If your gonna update, dont break it in the process