6.5 mm is just a more robust cable that is harder to break simply because it is bigger, however if you plan on taking your headphones out most phones, mp3 players, etc only support 3.5 mm. As for DAC/Amp depending on the headphones it doesn't really matter unless they have a higher impedance, it will give you better sound than the mob amp (usually) but it's up to you if you want to do that or not