Honestly, I find the loss of 1 AP/Level to be insignificant for the majority of the game and Archmage's 5% loss is just as insignificant (assuming you end at 300 AP, that's 15 AP lost from it or 33 total from by only going to 9 Offense). This means your Q at the end game does 20 less damage (20.4 with Havoc). Not using AP Quints is a bigger deal early-to-mid game than either of these, honestly.
Meanwhile, the extra points in Defense nets you -2 Damage/hit, +3% Movement, .45 CDR/level, and +3% Health/-10% CC duration. I grab Vigor for the time being (+3 HP/5) but I'll probably just swap it out for something else maybe.
You still get +3 AD, 4% CDR, and Spell Pen out of offense which is honestly the biggest bonus from the tree. The point of 9/21 is to greatly beef up her early game so you can harass better with QEW.
