I see the Seleucids as spiritual heirs to Alexander's empire. Like it says on the faction selection page, "Alexander knew". Babylon was the heart of his new empire, and Seleucia is the greater part of his legacy of conquest.

The EB Macedonians seem more like spiritual successors to Philip, Alexander's father. Their lands and concerns are in the Greek peninsula.

Pyrrhos might be his heir as a general though: he seems the most adventurous and mercurial of the successors, bouncing around from Sicily to Macedonia. He's the one who seems most likely to conquer a great new empire like Alexander, instead of inheriting it.