Octane uses both CPU and GPU unlike Iray, that uses GPU primarily
That basically means you can use larger scenes because geometry and textures will use your RAM to store it,
where power of GPU is oriented towards rendering the output only, which adds to speed.
Where Iray is more convenient though is that all shaders and lights are out of the box ready for rendering,
while Octane needs tweaking through the surfaces and settings most of the time.