All of the following testing was done on R52 Zahar'ak Halberdiers, at 351STR 304PIE.
For less wall o' text I uploaded my info here.

Not interested? Fine! Here's the tl;dr version, not exact, but you didn't look at the spreadsheets (there are two of them by the way, check the tabs on the bottom).

1 attack power ~ .38 damage increase overall. (.42 on WS)
1 base damage ~ 2.225 damage increase to Auto-Attack and Jump.
.01 DPS ~ .21 damage increase to Weapon Skills.

For all end-game content, GC Serpent is best, until you get the 5-piece bonus from Lieutenant gear (unless you are a Flamer and your lieutenant spear has Attack Power and a higher Base Damage on it by default…YOU [assholes] have the best spear in-game next to Garuda/Relic).

Yeah, you get more damage on Auto-attack and Jump with Ifrit, but for Primals and such, the majority of your damage isn't coming from those, it's coming from Weapon Skills. 15 more attack power and .27 DPS increase (at least for Grid) makes it worth it.
Average Doomspike w/Ifrit's Harpoon : 1047.5
Average Doomspike w/GC Sergeant : 1066

Ifrit:


Unsanctioned:


Sanctioned:

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