If you are looking to raid Emerald Nightmare the first week it is out, there are a couple of places you can look for answers. I've tried to compile some sims that answer some of your questions in the first post here:
http://www.mmo-champion.com/threads/...ting-All-specs
I'm sure Azor will be updating his guide for Legion full patch soon, as will most of the rest of us, but it's just a bit too early to write guides about things right now as we're still expecting some changes and we're also still looking for answers right now.
To answer your question about the artifact, there are essentially 2 schools of thought about leveling the MM artifact right now. The first is to just go after Legacy through DA and WG in 15 artifact traits and then over to Rapid Killing in 16 and Bullseye in 20.
(from the Theorycrafting thread linked above).
The second school of thought is to go straight after Bullseye, which is essentially like a really, really overpowered execute/kill shot, in 16 and then fill out to Legacy through WG in 20. You can see sims about this in the Theorycrafting thread and why this second option has the most dps in 16. The really important thing to remember here is that people who do the daily and weekly artifact improvement activities should have between 20 and 24 artifact traits when raids open. So, regardless of path you take, you'll have those 20 important traits with either of these paths. The first has slightly more dps when you are farming in mythic dungeons and don't get to really take advantage of Bullseye's ridiculous OP benefits on the longer kill times, where you'll be under its effects for longer (last 20% of boss health).
We expect MM to be the better raiding spec right now, mostly because being melee isn't an option for most existing hunters (we were recruited to fill ranged spots, and we expect melee to be significantly less forgiving than ranged in regards to mechanics, especially after the change announced by Watcher to remove access to position information from raid addons), and because the toolset available to MM is very powerful for both single target and AoE.
If you'd like artifact leveling information on BM or SV, please see the post linked above. Hope that helps.