As a Demon Hunter :
Reroll attack speed if you already have more than 2.5/2.6 Attacks per Second. At this rate you generate hatred fast enough and the difference between a 1.6 APS weapon and a 1.7 APS weapon won't be that much.
Reroll +8% Damage if you like having a high Attack Speed. This is my case. I have more than 2.8 Attacks per Second which brings a real comfort in-game regarding hatred generation and effect procs (like Night Stalker).
For the other classes :
Again, it depends on your build and your personal preference.
- Attack Speed allows you to : proc some effects more often (on weapons or skills), to crit more often, etc. With a Wizard for example, if you use the passive Prodigy, the faster you attack, the faster it generates Arcane Power.
- Raw Damage allows you to set up really high crit and damage. If your spells are not attack speed dependent like Ray of Frost you should go for %damage. The spell will take your Arcane Power more slowly and deals more damage which makes picking %damage over attack speed really important in those kinds of builds. On the same note, a fire Monk does not need attack speed but instead relies on the high damage the fire skills can output.
In the end it's your choice and both are good. You don't have to focus on the number displayed on the weapon but on the crits you actually do in the game and the frequency at which those crits appear. That's the difference between sheet DPS and effective DPS.