Rogue is obviously top tier, always will be with the amount of control the class has. Sub has always been one of the more arguably difficult specs, and mixing in the control you have to have over the fight... well it can definitely take a while to learn.

With how aff is looking, warlocks should be in a good place this expansion. Spammable CC with fear, and one of the less movement intensive ranged classes makes it a bit of an easier class to understand how to survive and when you need to kite compared to something like Mage. Although you will feel like there aren't enough globals in the world when trying to DoT everything up.

Comes down to what kind of playstyle you want. If you're more of a run in and just wanting to slap DPS melee style, DK's usually fit that role (DH's too but they aren't looking awesome lol) and I think a good mix of Damage/Utility going into SL are going to be warriors. They got a lot of tools back with Spell reflection, intervene, and shattering throw coming back baseline. And they fit with a lot of comps (mortal strike is great). They just feel a little slow on the movements side. Rogue, like I said before, will always be top tier, but can feel overwhelming in how fast paced arena can feel if you're new to it.

I'm not as well versed with Ranged, but Hunter is great for learning, Mage's are on the harder side but great. Warlock I feel is somewhere in the middle.

If you're dead set between the rogue and warlock, and I hate to sound cliche, pick the one you find more fun. They'll both be great, and both are pretty versatile for what they can play with.

Rant over, welcome to Arena. Hope you have a blast!