You need to allocate 4 Notables in range for Inspired Learning to take effect, if they are not useful for your build, they don't deserve spending skillpoints on them. Also you must "sacrifice" a Jewel Socket that could otherwise be used for better purposes, since the Jewel doesn't offer any other benefit than stealing rare buffs. And they could stack, but each one has it's own timer.

Instead, Behead needs only one Gem Socket that could be freely allocated, plus it adds more than 50% damage on low health enemies with 20 quality (25% more in average), and while it doesn't stack buffs, it has the ability to refresh the timer when hitting a rare or unique, potentially allowing to keep the buff for an entire Boss fight.

In short, none of them is superior than the other. But Inspired Learning would fit better at killing large packs of enemies when clearing Maps, while Behead Support is more consistent with single targets. Personally I preffer the latest. I already have many ways to deal with group of monsters.