EDIT: After several comments, both here and elsewhere online, pointed out that I have some incorrect information in the original post, I have edited it to be correct.
In fantasy roleplaying games, “Vancian” spellcasting is the default. This is well and good, because the written works of Jack Vance are fan-damn-tastic. Their influence on fantasy roleplaying are still felt today (You can find him in Appendix E of the 5E Player’s Handbook, which nothing more than the a lesser son of greater sires, *cough* Appendix N *cough*).
In particular, his ideas around magic are what have stuck in the game. A person can only keep so many spells in their head at once. Better users of magic can keep better spells, as well as more spells. Once used, they are removed from the person’s mind, until they can be restudied. A quick look at B/X, or even 3E, will shows you that Vance’s ideas about magic have stood the test of time in roleplaying games.
So whenever a roleplaying game comes along and does something different, my brain has to look it over two or three times to make sure I understand it. In ACKS, the mage character takes a small step away from the traditional Vancian magic system (as seen in B/X). At first I thought that was a bad idea, but I have come around to it.*
In ACKS, the mage may cast a number of spells equal to what his or her level allows, per the class chart. This number is also the number of spells he or she has in their repertoire, but the mage can receive bonus spells with a high Intelligence! The mage is not forced to decide to memorize one copy of spell X and two copies of spell Y or anything of that nature. They may cast on the fly, as long as the spell is in their repertoire.
For example, if per the class chart, a mage can cast / have the following number of spells in their repertoire, it would look this:
3 1st Level spells
2 2nd Level spells
1 3rd Level spells
That means this character has three different first level spells in their repertoire and they may cast three first level spells a day. They may cast one spell three times, each spell once, two of one spell and one of another, or any other combination.
..if the mage has 13 Intelligence (+1 bonus)
4 1st Level spells in repertoire
3 2nd Level spells in repertoire
2 3rd Level spells in repertoire
…if the mage has 16 Intelligence (+2 bonus)
5 1st Level spells in repertoire
4 2nd Level spells in repertoire
3 3rd Level spells in repertoire
…or if the mage has 18 Intelligence (+3 bonus)
6 1st Level spells in repertoire
5 2nd Level spells in repertoire
4 3rd Level spells in repertoire
This means that a mage has a reason outside the XP bonus to have a high intelligence. In B/X, it is better for magic users to have a high Dexterity (for better Armor Class) and high Constitution (for more hit points) than a high Intelligence. However, in ACKS, a high Intelligence mage has more versatility in the number of spells available to cast.
Due to not needed to decide in advance which spells will be cast in a particular adventuring day, the ACKS mage can have spells in their repertoire beyond the usual sleep, charm person, etc. They can branch out a little bit and have some spells available to use other than the “normal” ones expected of a mage. He or she has the option of having some situational spells ready to use if the party finds themselves in a pickle, instead of three copies of sleep memorized.
The ACKS mage can add a new spell to their repertoire, but they must either increase their level (and get more spells per the class chart) or exchange spells. Here is how that exchanging works:
Let’s say that the mage has, per the class chart and Intelligence bonus, 2 1st level spells in their repertoire. Let’s call these spells A and B. The mage finds a new spell called X. The mage wants to use X, but with only 2 spots in the repertoire, the mage cannot just simply add X. Therefore, after spending a lot of in-game time and gold , the mage removes A from the repertoire and replaces it with X.
The mage’s repertoire now contains B and X. A is not 100% forgotten, just not part of the current repertoire. If, at a later date, the mage decides to swap A back into the repertoire, they may do so, by expending time and gold, just as before.
Thematically, the repertoire concept lends itself well to the trope of the wizard sitting in their tower for long periods of study.
Why is the wizard doing that?
To make changes to their repertoire!
After the long study, the wizard can exit the tower and cast the right spell for the situation at hand. Which, once again, is nice thematic touch.
In conclusion, the ACKS mage has more versatility than the B/X mage and a noticeable, but not overwhelming, increase in power due to this versatility. B/X is a fine system, but more and more, I think ACKS might be just a bit better.
* I am aware that elves exist in both games and have the same rules for spells, so everything I wrote here can apply to both.
So why not mention them?
Well…my thoughts about elves can be seen here (NSFW):