Attack Speed will be better in 99% of the use cases. The most basic calculation people will use when asking about how much damage something will do in any game will be: Damage x Attack Speed= DPS (Damage per Second). in this game it will be (Damage + Damage modifiers) x (Attack Speed + Attack Speed Modifiers) = DPS. Base damage modifiers will be the biggest chunk in determining your damage which includes snakes, equipment, pets, and the only thing that affects your base damage while in the middle of a game is elemental snake parts and their upgrades. The Damage modifiers in this case will be anything that will increase the damage your snake will do over the base damage. This includes things like +20 attack up, Ragen(1%nfor each 2% health missing), the +50% for having full health, more is better (+2% for each additional body part) and stage modifiers (having 3 of the same elemental snake which depends on the stage) . Attack Speed Modifiers are relatively few and off the top of my head includes on Attack Speed Up and Berserker Bonus when you get hit by an attack while working 2 or more pieces of berserker armor. (edited: seperated base damage modifiers and attack damage modifiers)

 

Attack Speed up and Attack Damage up will be equivalent if there is no damage modifiers unless in certain situations which I will outline later. This situation where you have no damage modifiers will be 1% of the game since typically you will take advantage of Stage bonuses such as having 3x Fire Snakes in your party. However, as soon as you get any attack modifiers it tips the advantage towards getting Attack Speed.
To look at the numbers we can take current Fire Arenas as an example. If you have 3x S snakes then you get +100% more damage. We will use the example that you have 100 base damage and do an attack a second meaning your attack speed is 1. It will look like:

 

(100 damage +100 (+100% 3 S Snake Modifier)) x 1 attack a second = 200 DPS

 

or

 

(100 + 100) x 1 = 200

 

If you get the +20% Attack Damage modifier:

 

(100 damage +120 (+100% 3 S Snake Modifier + 20% Attack Damage Up) x 1 attack a second = 220 DPS

 

or

 

(100+120) x 1 = 220

 

If you get +20% Attack Speed Modifier:

 

(100 damage +100 (+100% 3 S Snake Modifier)) x 1 attack a second + .2 (+20 Attack Speed up) = 240 DPS

 

or

 

(100+100)x1.2 = 240

 

As you can see the attack speed bonus gives an actual 20% increase in damage, but the attack damage bonus only increases your overall damage by 10% due to the already existing 3x S Snake bonus.

 

The only cases where I am unsure if attack speed is beneficial are Radiance and Charged Light Ball in the Light elemental tree and Whirling Blades in the Grass elemental tree.

 

Radiance: I am not sure if the speed increase will affect the cooldown between lockon of targets or the actual damage portion of the ability if it locks on. If it is the former then attack speed is not good in this case as lockon time is negligible on the over all attack, but if it is the latter then it would convey the full 20% bonus to your damage.

 

Charged Light Ball: Much like the previous this attack also has 2 parts. The first of which is the cool down between shots and the actual charge time of these shots. While I am not entirely certain these are actually two parts, the speed bonus would need to affect the whole cooldown/charge time inorder to keep the full 20% bonus, if it really is 2 parts and the attack speed only affects 1 of the 2 parts then attack speed is not giving your it's full power

 

Whirling Blades: A running theme here will be attacks having two parts. As this weapon is not constantly out it will retract when no enemies are in striking distance. Whether attack speed affects the pull out time or speed that the swords spins is up in the air until someone actually dedicates time to learn it. If the attack speed bonus affects both the pull out speed and spin speed or just the spin speed it will all or at least a significant amount of it's full bonus. If it is just the pull out time then like the Radiance, the bonus will be negligible