Personally I think Frost is just better- as Fire in 29 you're not going to have alot of chances to hardcast Fireball or Pyro, so you're stuck Fireblast and Scorch spamming, hoping for that 10% RNG stun. Fire also loses out on alooot of control talents like Frostbite and improved slow% on Chill. Cold Snap is another hugely important talent that can and will save you or your flag carrier. Having 2 novas is pretty nuts.
Another point is that, yes you're a mage so pew pew pew damage- but thats generally not your role in WSG. Poly, Blizzard (improved w/ chill effect), nova, rank1 Frostbolts, Counterspell- You'll be casting these more often than you will be sitting there nuking. The gear options are heavily skewed toward raw stats rather than spell damage. That, plus all the OP physical damage in that bracket, makes mages very control oriented. You win by kiting and outlasting, not by ambush 1 shotting. The true power of a mage in this bracket lies in the ability to control all the melees.
Here's my preferred spec- there's 2 floater points that can go into Improved Frost Nova, Frost Channeling, Arcane Subtlety. Maaaybe Frost Warding, maybe a 3rd point in Imp. Blizzard with the last point in whatever
https://classic.wowhead.com/talent-calc/mage/--05203030122
For reference, here's the gear i'll be using right when BG's hit. Depending on If you're Horde or Alliance there's a couple interesting sidegrades possible. I dont know how to link items so i'll just list them. Gloves- Hotshot Pilot's Gloves (Super rare Gnomer BOE). Belt- Warsong Belt (Horde). Boots- Acidic Walkers (Gnomer BoP). Mainhand- Zealot's Blade, Offhand- Omega Orb (Horde, Troll mage only). Rings- Deep Fathom Ring (Wailing Caverns), WSG Rep Ring.
Hats- Tons of options. If you're Gnome- Goblin Rocket Helm, Deepdive Helmet, Rose-Colored Goggles. Non-gnome- Spellpower Goggles Xtreme, Catseye Ultra Goggles, Lucky Fishing Hat, Fire Goggles (+17 fire res, not that useful)
https://classic.wowhead.com/gear-pl...o3g04sA1kLB_cMJXQNVD4OSaLPJoROLwNTUAxxA18SG1k
I know you only asked about spec, but i figured why not add some info =)