I don't know about walk behind lawn mowers, but my brother in law has a commercial Cub Cadet zero turn and I have a Toro ZTR (purchased from home depot). The internal parts quality like the engine are pretty much the same. Just a slightly bigger engine of the same make. They even use the exact same air filter. The differences is in the outer shell. No plastic on the commercial unit, has a larger cut diameter and an engine guard is standard.
I don't think it matters all that much for a residential cut, but I do take pride in the build quality aspect. Reminds me of my older John Deere (residential) riding lawn mower. But my older JD even has a superior engine to the commercial unit. They don't make residential lawn mowers like they use too.