I have both and prefer the sound on Open version, Closed sound more muddy (it's not as clear sound - lowering bass in EQ helps a bit).
Comfort wise, the Closed are way ahead, that's why I have ordered some mesh memory foam ear pads from Geekria on Amazon. Open have normal foam which is stiff and not as comfy and not breathable too! My ears get hotter in the Open version.

Be aware that Closed version is a bit heavier than Open, personally I notice it straight away.

My previous headphones were Sennheiser HD560s and I must say H6pro Open version comes close although HD560s are way better for listening to music, for gaming (any game) I prefer EPOS H6pro - comfier + microphone + okay sound too.