Well, Gnomes used to be the BiS race for alliance casters back when Expansive mind still gave us an intellect boost, that´s why many of the ones you see still running around are warlocks, priests and mages.
With current racials , we get a 1% haste boost from “nimble fingers”, and our “escape artist” racial is primarily useful in PvP, but also has occasional PvE uses (For ex dispelling roots on Tindral and Ansurek, just to name 2 recent bosses), and “Expansive Mind” bow gives 5% additional max resource (mana, runic power, Focus, etc…)
Mechagnomes on the other hand have their baked in mirror images which are IME mostly useless outside of singleplayer /open world PvE content, and get a 1% crit buff.
However, they also have, on a 2 minute internal CD, “Emergency failsafe”… dropping below 20% health instantly heals them for 15% of max health
More importantly, they have “Combat analysis”, a stacking mainstat buff (10 stacks max) that decays out of combat.
The last 2 are what makes them overall arguably one of the strongest alliance races for raiding as well as perma-chainpulling in M+ where you aren´t exiting combat every minute or 2.
So, the question is also “what exactly do you want to do, and do you care about min-maxing”… i.e. what are your plans for the toon. If you just want to bomb around in the open world, HUnter or warlock, if you wanna be a farmers plow create a fury and equip 2x armageddon (Pullhorse not included)…
For ex my Mecha is a rogue called Canopanah, because she exists purely for the purpose of opening lockboxes and transporting fireworks fron my Bank to the AH in december 