ACARS is an old (fashionned )character orientated communication system owned by the airlines ( ARINC/SITA)whose messages have to travel half way around the world before it reaches it destination (Singapore or Atlanta )
ATN is a digital bit-orientated sytem which whose messages will be transmitted via ATS providers or Satelites\links and will be quasi instantanous.
The difference is about the same as between CD and US 8-tracks audio cartrides.
I leave it to you to find out what the future system will be.