Generally, I encountered three big systems for limiting the amount of magic that characters can wield at a given time in my RPG career. Grossly simplified they belong to one of three groups: Mana-Systems, Backlash-systems, and Magic Slot-allotments. Of course these come at different names in other systems, but their ideas are there
Pay the Price-Systems aka “Mana” et al.
Mana systems generally come with a set number of magic points that can be spent on spells. Most spells take several of these points and balance the used amount with the power of the spell: the more powerful, the more points it takes. For example the-dark-eye-4 uses that approach. In call-of-cthulhu your price is sanity, making it almost-mana, and the life or stamina of a character also can step in as mana replacement in many other systems.
I think they offer a huge variety, as you can drain that mana pool with a huge variety of setups, but their main drawback is bookkeeping.
When most spells only cost a single point an that is not linked to a vital statistic, the system becomes pretty much a Magic Slot.
Magic-slots usually represent a single cast of a (baseline) spell. This makes tracking easier, no matter if you need to choose your slots first (as a dnd-3.5 wizard needs to) or can spend them willy-nilly as a l5r-4e shugenja. The downside I see is that we don’t get the same balancing fine thread screw as in a Mana-system but at increased usability.
Some variants also allocate levels to slots to try and balance the power of spells to their usage.
Fighting the Backlash
A totally different approach is gone in shadowrun: you can cast as much as you want, but each spell kicks you in your shins and can damage you. The premise here is, that magic has a Price of Power (TV-Tropes warning) – but there is a mechanic that allows you to resist that damage. It is somewhat related to Mana-systems in that you pay for magic with a statistic (typically health, stamina or sanity) but got a chance to resist that loss.
The downside here is clearly increased rolling but at the benefit of occasionally more magic. Or less. Or actually giving magic a proper risk.
The Balancing act
Now, with the basis out of the way:
What design maxims should determine a choice between those three systems (or alternatives)?
Are there inherent factors that make a magic-limiter system fit better to a specific approach to designing a game than others or does a specific magic-limiter system grow naturally from a specific approach to gaming?
Example questions that might help to answer what counts as a design maxim for this question:
- Might X-system better suited for a simulatoric/dynamic/whatever approach to designing an RPG than a Y-system?
- Can a goal of how the game is to be played or what stories should develop be the factor to decide what kind of system to choose?
- Might dice/game mechanics make one system a superior choice over others?
- What considerations in streamlining the game mechanics can sway the needle between magic-limiting mechanics?