A 40mm cannon would have a higher rate of fire, higher penetration, better distance, more accuracy and a more varied type of ammunition.
For example a 40mm cannon can shoot APFSDS (Armour Piercing Fin Stabilized Discarding Sabot) Rounds which could penetrate many armour types. It could also shoot HEI (High Explosive Incendiary) Rounds as well, which would have the same type of effect as a grenade would. A 40mm would also be able to accurately hit targets at ranges of around 2000m. More of a direct effect.
Were as a 40mm grenade launcher, in Canada at least, really only uses one type of ammo which is a HE (High Explosive) Round. The M203 doesnt really shoot that far (400m ish if I remember right?) is very much an area weapon, and is slow to load. Of course a mounted C16 GL can shoot sustained fire at further distances, but will still not be as good as a 40mm cannon for hitting armoured targets. It would be good at suppressing large groups of infantry and columns of soft targets such as supply trucks.
Hope that answers some of your questions. Both are useful in different ways.
As for my experience and knowledge... I was an C8/M203 gunner overseas, and a 25mm gunner qualified.