My understanding is that they are both equally important. A unit has certain intrinsic values that are modified by both experience and effectiveness. For example, if a unit has 4 guns and 70% efficiency and 50% effectiveness; its effective guns value will be 8 X 75% X 50% = 1.5. The same as a unit with 4 guns and 50% efficiency and 70% effectiveness.
BUT if someone were to say which would you rather have for me it would be effectiveness because the lower it is the morelikely your troops are to be overrun. At least that is my experience.