Does it actually matter? Both specs in hands of a capable player do pull similar numbers. Differences mainly lie in the toolkit and mobility, which a good marksman still can compensate in most cases. I have been playing MM since the start of legion and wont switch to bm no matter what happens. I just cant stand BM, since to me it seems just braindead after playing the legion-marksman for quite some time now. BM gameplaywise really just is a melee that doesnt have to be in meleerange. I cant get myself to like it. All the mobility they took from the Warlords MM Hunter was virtually doubled and thrown onto the BM. Not that I'm all to salty about it. It was kinda fun while it lasted, but again imo Hunters shouldnt feel like a melee-dps (except for Survival obviously). What actually bothers me that Marksman gets screwed heavily by legendaries (not that most other speccs wouldnt suffer from bad legendaries). Where they more or less fixed BM's resourceproblems by making the shoulders baseline they yet ignored that marksman to some extend is too stationary. Could have baked the bracers effect into the specc to ease it up on heavy movement, since without proccs (or the bracers) mm is kinda screwed if he has to move around a lot especially if you are trying to get the most out of t20 2p.
And who really cares about what rogerbrown and those guys play? Would you really switch to a specc which you might not even like just to have a pseudo-advantage which isnt even needed except for maybe in those Top-1%-Guilds? People tend to forget that there is mythic raiding and hardcoremythic raiding to claim world firsts. If we are talking about "serverfirsts"... lets face it there are a hell of a lot servers out there which have exactly one or two guilds left which are able to clear the content in time so there is not really any competition to worry about, and how many here in this thread actually participate in the real "wirld-first-race"?
TL,DR:
Play the specc you want. Both are competitive enough to not be a bother for your raid.