INT Vs Magic Damage

Working on my Keraunous weapon and was wondering for the dusk stone part, should I be trying to get INT or Magic Damage. I presently have INT +18 which I know also helps me a bit defensively but is this going to produce good enough damage for doing magic bursts?

INT

INT is generally superior to MDmg in equal quantities, as dINT is multiplied by some amount depending on its exact value and the spell in question. This value is usually >1, especially for the higher tier spells that a BLM favors. On weaker spells (T1/T2, helices) that multiplier may be 1 or even 0 at high dINT, but that's not worth building your weapon around. INT will also contribute a bit of magic accuracy.

INT doesn't add any damage to Death, but there's a cap on mdmg for that spell and you still get the extra macc.

That said, Keraunos is a bit dated nowadays. I'd worry less about getting a perfect augment than getting a newer weapon that will outclass it regardless.

When it's JUST +X INT v +X Mdmg, like your issue of dusk stone augment, then INT will likely be better and here's why:

Firstly, I have to qualify the "likely" because I don't know what your gear is, so it's possible that you'll get larger maxes from Mdmg... But overall, INT would still be better due to the first real point:

#1) INT = Macc&Mdmg. And that Macc is where you'll be getting the better results from. Because it doesn't matter if your max damage is 99,999 if you're getting maximum resists constantly. (That's like 6,000ish damage iirc.)
And with this endgame, everyone needs all the Acc/Racc/Macc one can get.

#2) While INT's conversion into Mdmg is variable (since it's ΔINT based) and is also a hard capped range, it's quite possible that your INT augment will be increasing the base damage by a smaller amount than a Mdmg augment.
However, depending on your gear and the target, it also may give you more.
It also depends on the spell...
But, again, INT is pulling double duty for increasing your damage, so the fact that you may already be capped or close to capped base damage on a T1 is moot enough due to being able to hit that full damage more often.

What other BLM staffs should I look into then? I hear Lathi is really good, but I won't be able to get that for some time.

nibiru staff is a good option. Drops from Umdhlbi in Escha-Zitah. Can be augmented with Magic Accuracy +25 or Magic Attack Bonus +25.

Lathi should be your goal as a BLM these days, or a perfectly augmented Grioavolr.

Lathi drops in Escha Ru'Ann and is augmented via Eschalixirs on one of 4 paths so is MUCH more predictable.

Grioavolr drops Bashmu in Reisenjima- a much harder mob and requires random augments from Taupe/Fern/Pelllucid Stones.

Many shells are selling these staves these days as they are capped on them. If you can personally get the pop items, it shouldn't be too hard to find a shell or a few ppl to mercenary either mob (but in particular Yilan for the Lathi- he also drops BLU sword, RDM sword, and some abjurations that you might be able to barter with)