The key difference though, is similar to how Authority's policy gives you a lever for happiness (need happiness...add a garrison!), this change offers you the same concrete lever for progress (need happiness....build a library!). Now strategically you may decide to delay that happiness in one of your cities for another tactic, that's your choice...but the bonus is there, all you have to do is take it. With the growth numbers, you can't just "take it". You can "help it along", "nudge it slightly", but you can't just take the happy when you need it. If your city is far away from the pop number, there's simply nothing to do but wait.