Not sure?
Try it yourself! make yourself a test map run it multiple time count exactly how much damage difference there are between the two spells in all your runs and make an average.
Based on my empirical testing here is what it looks like:
d&d does 40% more damage to farms and towers than blizzard and that is very significant d&d will kill farms that blizzard does not.
Blizzard does about 20% more damage to ogres but it also is way more randomly distributed than d&d meaning 5 cast on a pack of ogres will not damage all of them just a bunch of them.
It means a lot of the dealt damage is wasted if the shards do not perfectly hit the ogres, and that is a lot more random with blizzard than d&d.
It is also worth mentioning that an ogre or knights standing directly under a cast of either spell wont survive its full duration.
Also a hasted d&d on a pack of ogre will kill them faster than a blizzard and will cost way less magic than multi-casting.
Both races can multicast it is not a human thing.
I think any honest and somewhat reasonable player would always rather have haste than invisibility the spell is just cheaper faster to research and can be used in a wider range of situations, it is just a stronger spell.
With invisibility you can sneak a mage and kill some peons mining can't you do exactly the same with hasted dks? Wont that hasted dk do even more damages?
Ever tried using invisibility to blast a pack of 9 ogres by surprise? Well it works with a hasted dk humans can't compete on that part either.
How often can you use Invisibility? It costs 200 magic to use, it is just as much as polymorph or two death coils, 4 haste, it is worth 8 casts of blizzard...
A mage starts up with 85 mana meaning you have to wait a looong time before you can use that spell, you do not have to wait for haste and it will not have to wait long to use it again.
Sure invisibility looks cool and is fun to play and watch or use but it does not have the impact haste has it just is not as effective.
On hsc transports are fast, what are the odds you will be able to stop a transport with a hasted dk instide to kill your peons/farms/towers?
Also a good thing to note on hsc late game people tend to "slow mine" with only a few expandable peons what do you think hurts them more in that situation a hasted dk killing their farms and buildings or a mage killing these few mining peons :-)
So no Humans are not at least even they are straight bad! Very very very bad!
If you still want to argue blizzard is not inferior to d&d I will leave it as an equal but keep in mind the rest looks like this:
heal < lust or runes
exhorcism < lust or runes
slow < haste (same price)
invisibility < haste (cheaper and free if you already spent that to get your slow counterpart, 2 spells in one)
polymorph < coil (It is better and it is free vs 2k gold 200 ticks to research)
ballistats < catapults (no loading / reloading time before shooting)
^all of the above I have backed with detailed explanation before and proper comparisons.
I dont even bother mentioning the other human spells because they are just not even worth using.
On top of that orcs have unholy armor and whirlwind that do have a use in game.
And just like Lone said there is no advantage for that cost difference on the weapon upgrade if anything it is a disadvantage on maps where the gold is even more rare like POS low.
Gold is just that more valuable than lumber in almost all maps except for water maps when you go water big time in which case your are most likely to never get that upgrade anyways.
The only advantage humans have is for the marksmanship upgrade vs trolls regeneration that no one ever does anyway because it is not worth getting in the first place.
But there it is here is your big humans advantage: marksmanship!
So yeah game is not balanced, humans suck and honestly no one cares because the imbalances are not game breaking, you can still win as humans and most strategies can be played with them with fairly the same result and if they were played by an orc.
Conclusion just like baRa said the fact that humans are weaker than orcs is a fact and is in itself it is fun, it makes them challenging and a good way to play with a handicap against weaker players.
Since the game does not have a handicap mechanic like warcraft 3 or other games have it is one way to make that happen.
Just because you want to believe in something does not make it true if you want to convince people it is true, back your claims with undeniable facts and evidences.
Prove your claims don't just say they are at least even without double checking your facts.
Here ill even go ahead and attach a test map for you to try blizzard vs d&d for yourself.