The Problem with Slow Casting
Many of the answers to the prior question, mine included, suggest some sort of cost for using fast casting (a.k.a demonic magic). However, as the OP rightly points out, dark wizards would wipe the floor with mages in a direct confrontation; to win, the wizards simply need the initiative and comparable numbers.
Clearly, for slow casting to not be supplanted by fast casting, it must have some benefit. This benefit would not only have to mitigate the speed difference, but also make slow casting viable enough that mages could rely on it exclusively.
Why Bother with Slow Casting?
Both mages and wizards use incantations to bring magic to life. The words of the spells are used to define the effects the spell caster wishes to create.
But why do mages draw magic circles? Because they serve as a magical conduit, providing a steady stream of the kind of magic power needed for a particular spell to work effectively.
Also, why do mages rely on expensive materials? Because properly prepared materials allow the magic energies of a spell to bind to the physical world. This creates more potent and permanent effects.
By contrast, dark wizards simply pull magical energies out of the ether. This has a number of drawbacks, including the fact that the energy is unstable and will dissipate as quickly.
The Power of Creation versus The Forces of Chaos
Given the universes' inherent tendency towards entropy, destruction is far easier than creation. Any experienced spell caster can summon the magic needed to bust down a door, but only the methodical approach of the mage will work to put the door back onto its hinges. In other words, dark wizards revel in destruction while mages prefer creation.
Because the incantations are the same, a wizard's fast casting and a mage's slow casting can be used for the same general purposes, but the effects will differ dramatically.
* When a wizard uses fire, flames erupt in an unpredictable fashion, but when a mage uses fire, it only strikes the intended target(s).
* Wizards can create illusions that confuse, disorient, or distract, but only mages can create an illusion with enough substance and detail to be convincing.
* Mages can properly mend wounds whereas wizards simply cauterize them; if you care about your long term health, you'll want a mage as your doctor.
* If a wizard summons a creature to aid them, there is a constant battle of wills to keep that creature subservient, limiting the wizard's power; if a mage summons a creature, though, the creature acts in good faith, allowing the mage to dedicate his full attention to his next spell.
* Mages can create excellent golems and other constructs with considerable endurance and autonomy; anything created by a wizard will be nothing more than a fragile puppet.
* Likewise, mages can imbue items with permanent magical properties. 227
Anything a wizard makes will, at best, become inert and useless after a few hours; wizards with less patients or less experience may well create something that blows up after a minute or two. That's great for a bomb, but terrible if you want to equip a soldier with a magical sword.
The last bullet point is a mage's greatest strength. Artificing would enable smart mages to prepare for combat by creating a magical arsenal. Wands could serve as weapons to incapacitate wizards while amulets provide protection against wizard attacks. Rings might store precast spells or serve as miniature invocation circles for common spells. Materials may be condensed and stored within small capsules. There's a lot of possibilities. The key take-away, though, is that a wizard is unlikely to win a head-on battle with a properly prepared mage; instead, they need to rely on stealth, deceit, or treachery to gain the upper hand.
TL;DR
Dark wizards destroy and mages create. Wizards may be able to cast extremely lethal spells at moment's notice, but any sensible mage of means will create items so they are prepared; to succeed, wizards will have to use underhanded methods. 523