Since I'm looking at the same gun (just in two differnt calibers)
To me the gun is more important than 40 or 9mm.
If the gun gets too small then 9mm would be my pick, but I greatly prefer 40 in a gun that handles it well.
To illustrate my point... I own a PM9 and Glock 27 - each is well matched to its ammo - but I would not own a PM40 or a Glock 26.
Too much kick from the tiny PM40 and the G26 seems like wasted potential.