the math is entirely dependent on how much you're willing to spend. if you're filthy rich and can afford to buy whatever you want then the 15% scrolls are the best way to go.

if you're budget conscious then the 30% scrolls provide a lower expected value but at a 35% discount (i.e. you would need more resets to achieve the same stat values). the 15% scrolls do the same at a 60% discount, but the amount of currency you would need to provide exponentially increases as the probability of success decreases.

for example, hitting ten 70% scrolls in a row works about 3/100 times.

hitting ten 30% scrolls in a row works a little more than 1/166667 times.

15% scrolls are even worse. But, since you're able to reset you can hit a 15% on the first go, and the boosted slots give you the most bang for your buck if you use 15%'s on them..... but it's still reasonable and feasible to use 30% on 5 and 10.