They are certainly different focal lengths, and demand a different frame of mind when shooting. The 28mm gets lots and lots of context, though it also has the capability of singling out your main subject if you get close enough, changing the size relationship between your subject (close elements) and setting (background). In that way, it's a storyteller's lens, and requires a holistic treatment of the scene.
But that's hard, sometimes. I have said before, I get into periods where the 28mm just seems like an unintelligible foreign tongue to me. The 40mm, by contrast, is more here-and-now, more like "look at this," more of an editorial, snipped away part of reality. It's easier to arrange what you want to be in the frame because the edges of the frame are much more workable and compliant. You can fire off a shot more quickly and still come away with what you want.
At their best, storytelling shots with a 28mm wow me - see Garry Winogrand's work. But I consistently come away with better shots, at least better street shots, with 40mm.