long story short, what Xatu said pretty much sums it all up.
Let's assume you want to hit the fastest possible fpa with either bows, which are:
1. 150 ias with 08 wf (147 ias really, but for ease of calculation, let's round it up to 150)
2. 95 ias with GMB faith with 13-15 fanaticism aura
This means you MUST use an armor other then Fortitude if you plan to reach 150 ias with 08 wf.
This means you lose at LEAST 140% ed from armor portion of your gear (assuming you are using 160/60 armor), which is a significant loss of dmg, which in your case (a glass cannon i assume), is a huge comprise.
Overall, using Faith bow gives you not only more dmg, but also more flexibility in terms of other equipments you can use.
Hope this helps.
-korg