Mage or Priest

Try a coinflip - heads: priest, tails: mage. And see for yourself if you are happy with the outcome ;)

Puh, its a hard choice.

Mage, much gold, much fun, much pvp, easy leveling.
Priest, no gold, much fun, much pvp, harder to level.

Tbh. do you wanna Heal in lategame or do you wanna make damage? It is hard to find a guild who takes a Shadow Priest. So you should decide what your endgame goals are....

Honestly I'll just worry about the endgame when/if I get there. I don't think it's worth worrying about it until I get to max

But in this case after reaching max level you are basically stuck with your role. At least in regard to end game PvE. Would be sad to realize at that point that dps or heal role is not really your thing...

I know that but if I was playing a Mage I would be dpsing (ofc) and if I was a priest I would be healing. I just want to hear some fun things about each class

I was always fascinated with mage.
Fireballs and frostbolts! Love em alot! But I went with priest during Vanilla!

You know the difference healing or dpsing...

If you are always going to be with friends or grouped up Priests are very nice and very powerful, either healing or doing damage as shadow!
The thing I struggled with as a Priest was solo in the world, whether it was farming mobs or pvping. I kinda felt gimped.
This is one reason I am going to be Warlock in classic besides trying something new.

Mages can take care of himself out in the world when pvping and farming mobs.
They also have nice conveniences like creating their own food and water and teleporting around the world for quick travel.

One thing that always made me look at Priest when deciding a class for classic was the Benediction quest line!
That was the best thing to look forward to and it can be obtained early and for even casuals since its at the starter raid MC!
It will be BiS for most Priests and you can get it early and save your DKP for gear instead of a weapon!

Mages are a popular class as well as a Priest but having a good priest is better than having a good mage...being a good healer will always get you into a group for dungeons/raids and even pvp battleground groups!

I can't make this decision for you and I wont sway you one way or the other because they are both very good/needed classes in all aspects of the game.

Are there any major goals/achievements/fun quirks for either classes that could sway me either way?

What are YOUR major goals? This will determine which class you should roll. Do you want to heal or be a ranged DPS? As a priest, you will likely end up healing.

They are both strong classes in both PvE and PvP. Mage is arguably the most consistent DPS ingame. From dungeon dominance all the way through every tier, mages consistently perform. Priests are the exact same. This is the only similarity between the two classes, they are high performance classes.

For me it's an easy choice between mage or priest.

I hate the fact that mages rotation in PvE is chronically below 2 spells on average. At least I can get some variety if I go healer/shadow.

I also plan on WPvPing frequently so the ability to be versatile with healing/damage is a must.

Okay, I can advertise the priest here.

In PvE you can heal extremely well and it is also mega fun.
In PvP you are an extremely important role that should never be missing in a premade.

And if you don't feel like healen later on, you can just skill to Shadow and kill everyone in PvP.
I would do write even more now, but my English is not so good that I can write texts so long that I can transmit my fascination to priests.

I would decide based on what you wanna do the most in the endgame. As some have already said Mage in raids is very simple and their rotations could get boring as you are mostly spamming frostbolt/fireball. PVP is a completely different thing with mage having much more complexity there. It is easy to farm gold as a mage and you will be very powerful in BG's and WPVP. You have the advantage of making food, drinks and portals which will help you and others greatly. You can even stand in city and sell these services and make some nice gold as well. Portals are good time saver as well as other mentioned aspects of mage gameplay.

Whereas Priest is more engaging in raids and obviously worse for gold farming but not worst either. Priest is great for their spell diversity, you have a lot of flavourful spells such as Mind Control, Mana Burn, Dispell Magic (both offensive and defensive), Levitate, Mind Vision etc.

If you are mostly looking for the most fun raiding experience then I would go with Priest and if you value high self-reliance in the open world then I would go with Mage.

I know that but if I was playing a Mage I would be dpsing (ofc) and if I was a priest I would be healing. I just want to hear some fun things about each class

Priests can mind control enemy players off the lumbermill in Arathi Basin or if you're really mean you can mind control them off the Booty Bay ship.

Frost mages are very fun to play in BG like Alterac Valley but also a little squishy.
I've tried healing as a druid in Alterac Valley and I've got to say I enjoy freezing everyone in Alterac Valley a lot more.
There's nothing more annoying than being stuck in ice but It's fun when you're the one doing it.

Priests can mind control enemy players off the lumbermill in Arathi Basin or if you're really mean you can mind control them off the Booty Bay ship.

Or if your really, really mean you can mind control them underwater and drown them! (As long as their not undead :mrgreen:)