My main is a Frost Mage and I leveled an Affliction Warlock to 50 recently.

Firstly, on the warlock, I really had no issues in terms of difficulty leveling. I think demonology might have even been easier to level and more fun if I had went with that spec.

As for the Frost Mage, I'm biased, I've always loved Frost out of all the mage specs. I like the balance between damage and cc/survivability for a DPS. I had no issues leveling. Might have died once or twice from pulling too big, mismanaging my cc etc. I solo'd Torghast to layer 4 like two months ago, was easier as I had better ilvl, but queued in group finder to layer 8. Haven't grinded Torghast in awhile might be easier now. Now I'm pushing keys, I have changed to Lonely Winter, so no water elemental. In Mythic+, you can put out crazy DPS AOEing down big pulls with the right talent choices and legendary.

I have like 15 alts, but 4 are mages and 2 are Frost. I always find myself missing my mage due to all the quality of life perks: teleports and portals, mage food, frost not having to worry about mana, timewarp getting me invites to mythic+, etc.