For skill order, I prefer to get just one level of FF. Increasing it does make it do a little more damage, but it also increases the mana cost by quite a bit. And at low level, that is a huge deal. The stun duration is not affected, and the damage is significantly greater from a higher level of FB than FF.
1 point in FF at lvl 1, then max FB as soon as possible, and dump points into the wall next. Then I get FF last.
To each his own.
edit: Also, I have been liking a 0/9/21 build on most casters these days. Well, if I'm getting a tear on them, that is. Strength of Spirit is like having a mini Force of Nature to heal you up, which is amazingly awesome on casters. It prevents you from going back to base to heal up after every scrape.
I think the 15% little extra penetration is more than made up for by the health regen, which means you can stay out longer and farm more, plus the extra armor and magic resist do make a difference.