All models aside, I think the way that Mortuss explains it makes the most sense:
Lets take the turn 4 as an example again. The player with crystals could have invested up to 46 mana into threats so far, while the player without crystals could have invested 50 mana into threats. I think its obvious that 50 mana can buy bigger threats than 46 mana.
The amount of mana a player can spend makes more intuitive sense to me as a resource metric than the amount they can channel, even if the two are equivalent. Devoting 5 mana (and a quick action) to a mana crystal means you have 5 less mana and 1 fewer action to spend on other spells. The crystal player is 5 mana behind on the turn that they play the crystal, 4 mana behind on the next turn, and so forth. The mana crystal is easy to take into account because it only has one significant impact on gameplay, which is the channeling boost. And there is simply no way to deny that until 5 channeling phases have passed since the casting of the crystal, the non-crystal player is able to spend more mana than the crystal player, and has an extra quick action to do so.
DaFurryFury, I'm afraid I don't understand your logic or this idea of action potential. Mage Wars has 4 resources - cards, actions, mana, and life/damage. Mana crystal decreases your mana and repays its cost over the next 5 turns. It also consumes 1 action. It has no other benefits. In terms of economics, resources now are worth more than resources in the future, so it arguably takes even longer than 5 turns to pay back.
I think if you want to make your point, you will need to provide an example of something that you can do with a mana crystal (within the first 5 turns of casting) that you could not do had you not cast the crystal, because it has already been pointed out that there are things you can do without the crystal that you cannot do with it. If you cannot find an example, it means that during the first 5 turns, not casting a crystal is better, since the options it provides encompass all the options the crystal provides
and more. You need such a counterexample to make your point convincing.