Venomoth, in my opinion.
Even though Masquerain has Intimidate, Pokemon could still go Special on him and take him down. He has that 4x Rock weakness. Easy OHKO. However, Masquerain has a slightly better Defense (2 base points higher)than Venomoth
Venomoth has better stats, and as you said, has access to a sleeping move(Sleep Powder), a confusion ove(Supersonic), and Atrract, making your Quiver Dance setup a LOT easier. Venomoth doesn't have a 4x weakness like Masqurein does and it has a better SDef, helping it survive a potential OHKO.
So, once again, I would go with Venomoth.