I researched this a bit awhile back and went through a pile of old posts. The general consensus was 50/50 on which would cut faster. Even though the 3/8 chain has more cutters, the chain is moving slower since the rim is smaller in diameter in 3/8's. So I ran some numbers to see if the smaller diameter rim of the 3/8's chain, running more cutters, would actually pull more teeth through the cut than the .404 setup.
I won't bore you with the math, but on a 24 inch bar with the saw running 12,000rpm, it will pull 41,200 cutters through the cut (38 cutters on the chain) in one minute with .404 chain, and 42,420 cutters through the cut (42 cutters on the chain) in one minute with the 3/8 chain, both running 7 pin rims. That is so close, that the difference could simply be a slight error in my measurement of the chain.
I would imagine that it is probably more dependent on the saw (hp vs torque) and the type of wood it's cutting. I could be wrong, but I would bet you could run some live tests with different saws and different wood and get results going both directions.