Cyclone - Attack Speed or Physical Damage?

Which one would be better if I had to choose between the two?

For DPS the scaling is the same as with any other Attack skill - it depends on what you already have and the values of what you're getting.

The only difference between Cyclone and other skills is that Attack Speed affects kiteability on those other skills while it doesn't on Cyclone, so it's more likely for physical damage to be better there (since presumably the game is balanced around Attack Speed providing less damage but more utility).

DPS differential for Attack Speed is easy because it's listed in your character sheet; in the offense tab, for the skill you click, you can see "Attack Speed Modifier." Then the math is easy for the "more DPS" marginal benefit:

(100% + LISTED + BONUS) / (100% + LISTED) = more DPS

For example, if I see 50% under Attack Speed Modifier when I press C, and I want to know how much "more DPS" I can get from adding 25% increased attack speed, it's:

(100% + 50% + 25%) / (100% + 50%) = 1.1667 = I get "16.67% more DPS" from 25% IAS.

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It's possible to do the same thing with increased damage (whether physical, area, melee, etc.), but generally you can't find the total increased damage modifier without some kind of build tool. However, you can do some reverse interpretation if you happen to have a certain item that grants x% increased damage to your skills.

For example, you could buy a Carcass Jack. Carcass Jack grants 12% increased area damage, will be the exact same thing as 12% increased physical damage if your base damage is all physical (i.e. you don't have flat added elemental).

Take your Tooltip DPS wearing Carcass Jack vs. no-chest. Divide with-Jack DPS by without-Jack DPS. You'll get something like 1.05. Chop off the 1, and then divide the 0.05 by 0.12 (i.e. 12% increased damage). That's your marginal proportional benefit of x% increased damage for your current stats; you just multiply any Increased Damage % you have by that number and you'll get the "more DPS" number.

For example suppose your Tooltip DPS without Jack was 10,000 and DPS with Jack was 10,400. That means you got 1.04 for the effect of +0.12; so your adjustment should be 0.04/0.12 = 0.3333. And now suppose you want to replace Jack with something else, then you want to get 40% increased physical damage. 40% * 0.333 = 0.133, or "13.3% more DPS" from 40% increased physical damage.