This is a general aswell as personal question, I've been playing Horde throughout MoP and refused to go Alliance to atleast find teammates in WoD, don't want to make the same mistake again.
I'm going to play Demon Hunter in Legion and I believe Blood Elf is a better choice PvP-wise due to reliable silence, whether Night Elfs have Shadowmeld which requires a good timing and latency to properly dodge skills, but I don't really want to return to Horde.
Now, because of faction-based rewards for each season it kinda doesn't matter, but, as always, most people will aim for min-maxing and so do I.
I still think that Alliance racials are better in PvP, Humans arguably will be even stronger and as far as I know Demon Hunter favors melee cleaves and Humans or Gnomes are far better choice for other melee classes compared to Horde variants.
Almost unrelated, but I've been watching streamers lately and most of them play on Horde side on beta right now, of course on beta it doesn't matter, that's simply an aesthetical choice, but still.