I won't put all of the calculations up here, because they're inexact anyway, but here's how I estimate the difference between a Taru and a Hume in terms of casting power.
With the same gear, the Taru's INT will be, at most, 20% higher than a Hume's (no counting really early levels, the difference is tiny there). That's a lot, sure, but I noticed that when I, as a red mage, increased my MND by 12.5%, I got about half that percentage in extra healing power - from 27 to 29 per Cure. If INT works about the same, expect a Taru to be doing 10% more damage with an unresisted spell at his peak. At the end of the game, the difference in INT is smaller in ratio, so expect the difference to be smaller.
Looks to me like the real biggie is the MP pool. Still, use all that extra MP with a Taru, and see what becomes of you (splat). A Hume, from what I've seen, has enough HP to last for 1 more spell when taking hits from an incredibly tough mob. That can be useful in some cases (I take aggro from the WHM when the warrior's provoke isn't quite ready sometimes - let him take a whack, let me take a whack, back to the warrior, it all works out). So the Hume really has an advantage when it comes to overnuking. The Taru's extra MP means less downtime overall, which is nice.
So, a Taru can belt out sustained, conservative nuking, and have MP left over for debuffs. A Hume is better for those times when you just need to hit it as hard as you can and hope for the best.
Maybe this is different at much higher levels. Not like any of us have gotten that far yet, so don't even try to comment on it. There's still no better reason to pick one race over another than just feeling like it.