General

Shout should be a bind.

Even if Intrepid Slash looks good, you'll never use it again.
Go for Shout. It's the only AoE you'll ever get lol

I've been wondering why I never seemed to damage anything with only mobs in front of me.

Thank you kind Hero's. XD

You'll replace Intrepid Slash the moment you hit 4th job... so no point in maxing it.

Slash is useless at 4th job when you get raging blow
Shout hits more vertically and behind you than other attacks and can be useful in odd situations (like evo lab room 3)
Shout has slightly more value later because of it's hitbox