So I have 2 magic long bows.

IWD1 on elf fighter/thief with 19 dex as follows.

+2 Damage
+3 THAC0
+1Dex
Speed 6
I have 3 attachs with this bow equiped.

vs.

+1 Damage
+2 THAC0
Speed 3, Special: Extremely Fast
I have 4 attacks with this bow equiped

Which would you use?

I am leaning to the latter bow since I would have 4 attacks per turn, or at least thats what I assume that means.

I'd go with the 4 attacks. Another chance to hit and do full (or mean) damage is better than the +1 thac0 and damage, IMO.

Definitely the second bow for general use -- reserve the first one for enemies that require +3 to hit. Although the first bow is stronger (raw), the second bow will generally give you higher average (# of successful hits * bow damage) damage per round, since it's fire rate is far superior.

Consider ammo types for your bows as well.

The second bow just chews through ammo so I would only use standard arrows with that bow.

However, if you're running low on ammo or using enchanted arrows, I would use the first bow.

Use the second one, +1 to hit and damage is nothing compared to an extra attack per round.