TLDR: the reason why there is no clear answer, is because it does not matter that much.
There is a mechanic in the game that allows you to gather Artifact Power quicker the further we are into Legion. As you unlock traits on your artifact, it also becomes more expensive to unlock the next trait. So basically, you end up spending and gaining artifact power faster over time.
As the cost of a new artifact (be it on a different spec) are reset, you’ll be able to gain artifact power quickly on for example a resto or enha spec later on. So the real question is, WHEN does it matter? Because obviously all these people are concerned with something?
These are my two copper:
- Personally, I do not have the time nor the energy to bother leveling up a second artifact, even if it only takes 20 minutes a day for a few days to catch up. I have plenty of alts to worry about.
- If you’ve never levelled as enhacement.. all of the specs are viable for leveling. One is just a bit faster than the other. So why bother playing another spec?
- There will be a million people leveling, and a million mobs spawning. The chances of you actually having to kill a mob alone are pretty small.
- Just play whatever you want to. If you don’t already know the answer to the question ‘what spec should I level with?’, it probably won’t matter for your progression path either.