(This is purely a WSG standpoint)
A lot of players currently like DH as the no. 1 FC at 20. This is for the obvious reasons of its HP pool, mobility post 6 stacks, and Illidan's Grasp (eyes are actually not huge in a non-rogue meta, but obviously useful). I think that DH is rather self-explanatory and needs no introduction to anyone who understands vengeance.
However, despite the pro's that are mentioned, I'm still one of the few who truly believe that Rdruid is the superior FC. I'll explain my controversial opinion. Rdruid is factually (ask any DH FC in this bracket) better at picking and crossing with the flag due to stealth+dash+shifts, which translates to being practically better than vengeance for 0-5 stacks at the very least. This leads me to 6-15 stacks, where the FC and their kit becomes limited.
To start, I need to touch on a few misconceptions about vengeance DH. Despite their 10k HP pool, their effective HP is significantly lower as a result of taking 50% increased damage in PVP. Outside of metamorphosis, their overall tankiness is going to be much lower than a resto druid's as a result of their mitigation and relatively high HP (in addition to the option to pre-hot with cen ward and pre-iron bark). It's actually because of this that the strategy of vengeance DH revolves around avoiding damage rather than being a big pwn tank with their big hp pool. Additionally, it's because of their HP pool and increased damage that makes vengeance DH's significantly harder to heal consistent damage than a resto druid; basically the % of HP healed per heal is going to be lower on a veng DH than rdruid, despite being less tanky.
DH's kit will therefore allow it to avoid more damage than a rdruid, which logically means it's the better FC. However, I believe that if played at a high accuracy that rdruid will live much longer in a fair twink vs twink scenario. This is because of the concept of prepositioning. By positioning ahead of time and understanding the scenario in an in-depth manner, the need for said mobility that DH has becomes practically unnecessary. This isn't the main point, however. Even if an FC pre-positions perfectly, said FC can get dove at high stacks and die. This is especially the case in SL, where ranged slows run rampant in the bracket + 100% mounts; diving an EFC is simply extremely easy at the moment (requires HIGH stacks; 10+ usually). Thus, my point is that even though DH has its mobility- when kiting and getting slowed, said mobility will be equal to or less efficient druid shifting. Consequently, I do believe it's impossible to stop an FC from dying at high stacks over time. I'll refer to this concept of an FC inevitably dying as death-stacks (because it's edgy).
My next point is that rdruid's death stacks is going to be placed higher than a DH's in a situation that both players play at an exceptionally high level. I think that based off of previous information stated, this isn't surprising. However, a couple things should be said regarding DH and rdruid in regards to playing as efficiently as I stated. First of all, playing perfectly as an Rdruid FC is incredibly difficult; a lot of experience is required. If an Rdruid makes a mistake, it will likely be punished much more than if a vengeance DH made a mistake. Additionally, vengeance DH has the pro of playing off of an enemy's mistake post-6 stacks, unlike Rdruid which almost completely loses this threat at high stacks.
So essentially, DH is a much more forgiving FC than rdruid but will have its limits that are beneath rdruid's. Personally, I aim to play with as high of an accuracy as possible/as perfectly as possible, and therefore rdruid is the more attractive FC. I also don't mean to say that DH is easy to play as an FC, I'm simply saying that if played well it's going to be more forgiving; bad plays are still punished.
This was a long post and I probably got sidetracked or said something confusing. If you have questions, jUaN#3807 is my discord.
edit: I did calculate for DH's ability to hop up the GY; that is included in my definition of their mobility.
