Rod holder selection sounds trivial until you spend a day fighting a loose clamp or a holder at the wrong angle. The three decisions that matter are angle, mounting style, and material.
For trolling you want swivel holders with 15-30° of off-axis adjustment — this lets you spread lures behind the boat without tangles. For jigging or stationary fishing, vertical flush-mount holders keep rods out of the way and reduce fatigue.
Material-wise: stainless steel 316 (marine grade) is the default for saltwater. 5086-series aluminum is lighter and cheaper but needs anodizing. Avoid painted mild steel — it will rust within a season.
Mounting: gunwale clamps are removable and fit most boats but can loosen over time. Flush-mount requires drilling but gives the cleanest look and strongest hold. For aluminum boats, rivet-backed plates distribute load over a wider area.
Pro tip: measure your rod butt diameter (usually 25-30mm) before ordering. A holder that is 2mm too wide will let the rod rattle on rough water.