Only looking at 1 spec, it would be Destruction. Destruction will give you the tools needed to "carry" lesser skilled players in alt runs, where with Affliction, you have to trust your team to down the aoe trash, and short lived priority adds. Things that will actually wipe a group. Can't say I've ever had an enrage happen on a normal or heroic run, but I've seen add control go down the drain numerous times.
Why not both? If mythic raiding is your goal, you may as well learn and use both. Stack mastery enchants, gems, and with a few crafted pieces swapped from Crit/Mastery to Haste/Mastery, you'll be performing well enough with both specs for their respective fights.
welcome
you can do fine choosing whichever (not demo of course except for mythic manno) but you should master both destruction and affliction
affliction is best on single target fights with few to no adds spawning or council style fights (hellfire high council) and is also good on tyrant
destro is basically a jack of all trades. great burst dmg great sustained AoE and good cleave
picking up destro is pretty easy but mastering it requires some time
havoc is your most important tool learning how to shadow burn properly is what separates an average warlock from a good one
affliction on the other hand is pretty simple
keep your dots up at ALL TIMES
dont let soul burn haunt fall of (always try to keep at least 2 shards)
use haunt with darksoul or you have very high intellect procs (dsi proc)
drain soul as filler
destro mastery/crit 1%mastery = 3% dmg on chaos bolt and 1& crit = 1% dmg on Chaos bolt
with affli its a bit situation pure single target you want haste/mastery but still you need a lot of mastery
if at least one add is added then its mastery/haste so i recommend going mastery/haste since 12/13 bosses in hfc have adds
and if you have any questions the warlock community is the best and most helpful one there is
i hope i've been helpful
Good luck