I often get scolded for saying that demonic is a better damage item than wingbow, and that wingbow is a better tank item than demonic (and that consequently, they're miscategorized in the shop) for non-agi heroes.

UP FRONT TL;DR
(stats on a fully built Jeraziah)

Thus...
Wingbow = Tank item
Demonic = Damage item

Explanation:

Lets assume for comparison purposes that every hero has an average of 10 armor in a hypothetical game.

Damage reduction

With 10 armor, wingbow adds an additional 4.2 armor - this combines for a total of 46% damage reduction. If you take in to account the 30% evasion though, wingbow gives you, the grand total is 62.2% reduction.

With 10 armor to start, and 15 from demonic, your total reduction is 60%.

That 2.2% difference may not seem like a lot, but at 3000 HP, you can take about 200 more damage with wingbow before you die than with demonic. Also keep in mind, that part of the damage reduction of wingbow is evasion - meaning that chance-to-proc hits may be avoided entirely.

Attack Speed

Now for attack speed. Wingbow does indeed give you 5 more attack speed. But often times that wont affect your attack speed at all. This is a literal definition here, when I say it wont affect you at all, I mean that the 5AS boost will often increase your attack speed by 0%. If you have two heroes, one with wingbow, and one with demonic, and each are attacking a stuffed puppet over the course of an hour - each of them will most likely get the exact same amount of attacks in over the course of an hour. Why? Because attack speed falls into bins. A 163 to your attack speed is THE EXACT SAME as a 182 attack speed. It's not until you hit 183 (for most heroes - the numbers are slightly different on magebane and nomad) that your AS changes at all. Can it increase your AS? Sure, if that additional 5 breaks this threshold, but that will only happen around 20% of the time.

Effective Damage

Wingbow gives a nice 32 damage boost. Demonic gives none, but keep in mind that Demonic does a -5 armor reduction. Assuming the other team has an average armor of 10, that means you have an effective 24.2% damage boost with demonic. If you're sitting at 200 damage before you grab a wingbow or demonic, you'll end up with 262 or 232 damage with the wingbow, and 248 damage with Demonic. A clear winner for any non-agi heroes. But even with agi heroes, during a long match you'll often become fully built. At 256 base damage, demonic's armor reduction surpasses the damage boost that the agi from wingbow gives you. So for end game scenarios, demonic is often a better choice for damage output than wingbow.