Just to add on to what others have said, tempo and mana efficiency are related but not always the same, as you saw on turn 2. Playing Demon Companion would've floated a mana, which is not trivial, but would've given you an extra body on board at the cost of 1 health on your Initiate. Mana efficiency is more important when you have impactful plays lined up for future turns, which you didn't at that point (1 drop+hero power on turn 3 doesn't count). You can also talk yourself into believing that Demon Companion is worth about 1.5 mana since they're all overstatted compared to typical 1 drops.
And personally I only use QS as removal as a last resort. One for one trades don't favor you since their deck is so much more powerful. Ideally QS is something you use to get across the finish line when they're hiding behind a bunch of divine shield taunts.