I have been casually pvping, as in around 1800 rating since cata, but always healer or ranged. I boosted a rogue for legion fun. And have started gearing him up in pvp and having TONS of fun.
My main question is...
I play sub, and took some getting used to it. But i see a lot of 3v3 and rbg rogues going combat. Is it actually better? Or easier to "master"? Cuz it seems to me as sub, im bursty as all get out with dance up. So just curious the differences or reasons to play combat.
It is personal preference. You can do great things with sub (just watch someone like Pikaboo play). In fact, even assassination is viable at high ratings.
Combat dishes out insane damage, and killing spree is still quite strong. Combat is a bit more popular at high ratings on EU than US (especially after Blizzcon). US high rated rogues are mostly sub. I personally prefer the cool things you can do as sub. You can do all the crazy sap shenanigans so much more often as sub than combat. Shadow dance is such a versatile ability, and it has been the main reason people have even played sub in this expansion. It is going to be reworked in Legion, so I suggest you enjoy it while you can! (Then again, combat is also being reworked, so maybe just try both and see what you like!)
RBG's is a bit different though. I don't have too much experience with them, but I assume you would go for combat or assassination.
Short version is that combat is the superior spec for the most part, the damage/burst is much greater.
But indeed, sub allows for more control. You decide when you do your damage, not your red buff. You can lock down the entire enemy team during dance, allowing your partner to get his CC off easier to follow up. It's also easy to follow up CC with sap while dancing.