They are different, not harder. Both have far fewer lines than the Sicilian or 1...e5.
However, I should forewarn you that just because Black plays the same pawn moves does not make one opening even remotely similar to the other. The QGD and French are NOTHING alike.
The opening most in similarity to the French is the Kings Indian Defense. Aggressive Defense, Blocked Center, Attack is on the wings - the direction the pawns point indicate who should be attacking which side (French - W Kside, B Qside, KID - W Qside, B Kside), etc.
The QGD features the Static Center, not the Blocked center.
i noticed that i can play both c6 and e6 against both 1.e4 and 1.d4 as black.
after 1..e6, white almost always will play e4 or d4...same thing happens with 1...c6.
c6 leads to the Caro kann / slav defense, e6 leads to French defense/QGA.
i play 1.d4 so i already should know the QGD and the slavdefense....at this point my question is:
which is harder to learn, the Caro kann or the french ?