Both make the process irreversible. But, I think the interesting part of this is that, unlike the case of a frictionless piston with mass, if you have a massless piston with friction, then when the friction is very high (so that the gas expansion is very slow), your concept that the gas experiences an essentially reversible expansion is fulfilled (since the viscous contribution becomes negligible). However, even in this case, there is overall irreversibility to the process (due to friction), since the work done by the gas to push the piston is greater than the work done by the piston to push back the atmosphere. This is really an interesting problem that I think both of us learned something from.Yes, I get the same.
Does that mean that friction on its own doesn't make the process irreversible, whereas gas viscosity does?