Rogue can either take away the power or duplicate it can do both.
The real difference is that synch only has one power which is to imitate.
Rogue has several based in absorption. She takes powers and she has been limitless in this. Synch is not.
Rogue took the power of every hero on earth (and almost every omega on earth) proving she is superior to the overrated omega. That they were useless in that story is worth mentioning.
Rogue takes vital energy which with this could kill 'thor or apocalyose.
Rogue also takes or scans minds
Rogue takes emotions.
Rogue also physical and intellectual abilities.
The differences of the two are enough synch only has one power.
Rogue has several related to absorption.