It varies but ++ will most likely be better than +.

I say it varies since the game doesn't specify the exact values. You might find skills with 2 debuffs with the same/similar effect description but one is -12% and the other is -15%. You can find the values listed on the JP wiki.

http://xn--hckqz0e9cygq471ahu9b.xn--...8A%B9%E6%9E%9C