Well, keep in mind there's a requirement that you have someone at a certain rank before you can get those officer training boosts. I'm not as sold on Iron Will - my troops haven't really hard core panicked since I regularly could run people at > squadie level. I guess the way I look at it, given the choice between leveling faster and getting higher will that way versus just having more will gained per level, I'd rather get all the other perks from leveling up.
Though, that said, I finally had a bunch of mutons just spook some dudes with intimidate spam.