In our ACC we have both FANS & ATN equipped aircraft coming through. The difference is really quite large. FANS performance is very poor, very slow, sometimes just doesn't work at all. It's not reliable for our purposes. So much so we are prohibbited for using it for level change.
On the other hand ATN works a treat. It would be great if all aircraft were mandated ATN only, so this is probably good news for the single european sky effort.