Im aware that different *-archys have different rules for succession, but assuming the case in general for europe during the medieval era.
the son.
the type of succession for european monarchies you have a target image of in your head is one where the legal right is traced by direct lineage (grandfather to father to son). There are exceptions to this rule (say the early history of the British isles) but those exceptions are outside of the target image you're looking for and you're not really asking for a history of this sort of inheritance. When you go from elder brother to younger brother you're replacing one "line" of the family with another. The relevant differences in midieval claims is essentially the difference between agnatic and agnatice-cognatice primogeniture where the question is what lines count and take precedence (agnatic means only male decadents and their kids have valid claims, cognatic means all kids and ag-cog means all kid's lines but the male lines are prioritized instead of a strict chronology (youngest male then oldest female). The start of Shakespeare's Henry V is a fun illustration of this difference and its importance.
A good historical illustration of your question is the transition from Louis XIV to Louis XV. The linked tree is simplified but it shows how Louis XVI aka the great-grandson of Louis XIV by his eldest son took the throne instead of an older person from a more junior line.