Well, I think Observation Haki just requires a different focusing of the spirit. It might not be about how powerful the spirit is, but how you are able to harness it. You'll see an example in Dressrosa of someone getting Observation but lacking all other types of Haki. I'm not sure if you've gotten to that point yet. I'd assume that it requires more finesse and control, as opposed to Armament being more about brute force and willpower.