LogFAQs > #971291915

FFRK was a mobile game that replicated the FF6-style ATB system.
DFFOO is a mobile game that replicates the FFX-style battle system.
Of the two, FFRK was deeper and more engaging. You set up your party well, react to the boss, time your skills - it's like a mini-MMO boss.

Intellectually, they're not too different under the hood. FFX is basically ATB with the bar hidden and in permanent wait mode. The ability to switch characters is nice, but you're also limited to 3 characters on the field at a time. To be honest, a lot of it falls into the trap of "set up your team optimally and play rock-paper-scissors with the enemy"-hole that many jRPGs fall into. In terms of systems, I think FFXIII and FFVIIR are just obviously better when you dig deep.