Kante has better tackling, defensive awareness and strength, and a medium offensive work rate. So if you want to purely use him as CDM, no attacking contribution whatsoever, there is still a case for Kante.
Fred is faster, a better passer, better dribbler, better weak foot, better skillmoves, more aggressive, more stamina, better shooting and a higher attacking workrate (with 99 stamina). He's a beast and I can't wait till he gets his boost.
You can compare the two of them on Futbin for those who don't know (both on Shadow, chem ; 10, 100).