In D&D 5e, spellcasters often have to choose between spells that can affect a target in a successful melee attack. the fire bolt cantrip (1d10 to 4d10 damage depending on character level), for example, requires an attack roll, while the poison spray cantrip (1d12 to 4d12) requires a target Constitution saving throw. Obviously, this could apply class features other than spells that require a saving throw to avoid an effect.
Suppose two hypothetical spells that are of the same level, cause the same damage of the same type, affect only one (and can therefore be compared most directly with a single attack roll). Don't assume any resistance or immunity for the target, and assume the character is facing a creature from a CR corresponding to their level (as they would often do as part of a group of 4-6) . Also assume that players are experienced, but do not allowed to view Monster manual or other DM resources.
- Many experienced players will know the AC of common monsters, but even when they don't know it, it's easy to understand after a few attack rolls.
- Most players are often unaware of the full monster ability scores, which determine saving throws.
- Many DMs drive in secret. Even if they throw opposing save throws in the open, the fact that there are 6 potential save throws (and only 3 common ones) will make it difficult in a given fight for a player to determine the bonus of saving throw from a creature.
- Monsters have AC guidelines which are typical of a CR ("Monster Statistics by Challenge Rating", DMG p. 274).
- Many monsters have no saving throw skills. DMG recommends that save throw skills be used primarily to offset save throw penalties from low ability scores.
Given this, and other considerations provided in the response, from the player's point of view will a character succeed more regularly with an attack roll or a save roll, or are the results of attack rolls more predictable than save rolls?