akg k240

Are the new AKG K240s worth buying for quiet guitar and bass guitar practice?

is there a difference in the sound of the k240df and k240s?
I want to buy mixing headphones, somebody i know who run a good studio for years suggested me beyer dt990pro, but they are a bit over my budget, how do they sound different than the k240 ? do they really worth saving up?

thanks!