Spirit magic is the easiest and most ubiquitous. In most games, the player just decides what spirit magic they want, and pays whoever it is that can provide it them up to their CHA limit. Want Bladesharp 6? No problem (unless your cult forbids it to you). Just pay that coin to the priest of Humakt or Orlanth or whoever and get it. However, spirit magic is also the weakest magic, with the shortest duration and range.
Rune magic is common but requires more than just coin. You need to be an initiate of a cult and sacrifice points of characteristic POW to get Rune points. That’s way more expensive than coin! And when you use your Rune points, you need to offer sacrifices to your god to get them back. However, Rune magic is powerful, lasts many times longer than spirit magic, and usually has almost twice the range. And lets face it, the spells are just cooler and more exciting!
Then there is sorcery. To use sorcery, you need to study for years to learn the laws of magic. Think what it takes to be recognized as a mufti or a rabbi. You must learn to read and write, study the magical (religious) sciences, learn grammar, law, and all the rest of it. Maybe you get some kind of a certificate that ordains you as sorceror (probably common in Malkioni lands). In Orlanthi lands, the only folk with the literacy and the time to study sorcery are basically the initiates of the Lhankor Mhy cult (although many, perhaps most, prefer to rely on spirit magic instead). Now sorcery can be powerful, but it is slow to cast, time-consuming to learn, and expensive in magic points. But it does let you cast magic of whatever sort you can learn or devise.
See also
Of course there are other magic systems as well. For the illuminated initiates of the Red Goddess there is the weird manipulation of spirit magic called Lunar Magic.
The dragonewts have their own odd draconic magic system and it is widely thought that the EWF learned that form of magic as well. It is also possible that the Kralorelan magic is a variation of this.
While Spirit magic is the easiest and most ubiquitous, most player characters get their spirit magic by paying coin to their temple (lay man, initiate, or associate). Although shamans get the full range of spirit magic, they are not the main path to that magic in most RQ campaigns.