Order to choose is:
What you enjoy playing (its a marathon, not a sprint) > Space in raid group (going Warlock with 3 Warlocks in a raid group means you'll usually have to take a spot) > What is meta and good.
Other way results in many M+ Warlocks that have no clue what they are doing and fumbling and no desire to improve since they don't enjoy improving on that class.
Having a spec you're 10/10 to enjoy but very far off from the meta vs a spec you're 7/10 but very meta is a good choice. Choosing a 2/10 spec you hate that is also meta tends to burn you out.
To determine what's best in a meta:
Create tons of alts. Alts give flexibility, and even casual dropped alts rake in gold through missions, transmutes, access to new professions and dailies.
From the order above, see and start narrowing down closer and closer to the actual season. Note room for those specs - your raid group might get annoyed if you decide to play another melee in a sea of melee apps.
You really want to research and try out your choices. Don't just look at a tier list for 10s and call it a day. Ask around, look at streams, and look and see what is meta and how much you would enjoy playing it.
And be flexible. Last minute nerfs or buffs, a bug, etc. could completely upend a prediction.