I was wondering the same lately (mage or lock main for BFA) and this is what I came up with*:
https://imgur.com/a/6YWM7ea
Code:
mage vs. warlock
### 'comparable' spells category: ###
blink (15/20s) >>> demonic gateway (90s)
polymorph (no CD) >> succubus: seduction 30s
counterspell (24s) > felhunter: spell lock 24s
spellsteal (no CD) >> felhunter: devour magic 15s
remove curse (8s) >> imp: singe magic 15s
ice block (4min) > unending resolve (3min)
frost nova (30s) < shadowfury (1min)
invisibility (5min) > fear (no CD)
prismatic/blazing/ice barrier ? soul leech
### "no equivalent" category: ###
time warp
arcane intellect
battle rez (soulstone)
banish
### "don't care" category: ###
teleport summoning
conjure refreshments healthstone
slow fall unending breath (lol)
I consider all polymorph, counterspell, spellsteal and remove curse as superior as they don't require you to take suboptimal pet (unless they all do the same DPS now?) and are all available at the same time with shorter cooldowns.
The "don't care category" are things that wouldn't sway me one way or the other.
Not included in this table are:
- class fantasy (I like both)
- fire + frost + arcane vs. destruction + demonology + affliction (fun & ease of use)**
- frost utility in M+ in slowing/kiting
- pet vs no-pet class (ease of use in 'world content' vs. problems with pet AI? - is this still a thing?)
I didn't play mage or lock in Legion other than invasion-levelling, so all their specs are fresh/new to me.
* - all of this is subjective and potentially uninformed as I haven't played in some time, feel free to correct me if I'm wrong; also note that this is PVE perspective as that is what I intend to play mostly
** - fun is subjective of course; 'ease of use' - after maining shadow priest for a few years I refuse to play a specc that requires me to put in way more effort than other classes just to get their results (DPS)