Most material advice starts from the material and works toward a use. This works the other way: state what the part has to survive, and get a shortlist of materials that can survive it.
A shortlist rather than a winner, because most requirements have more than one reasonable answer, and the tiebreaker is usually what your printer can do rather than what the polymer can.
The questions that actually narrow the field
Service temperature. The single most decisive constraint, and the one people underestimate. PLA gives up between 52 and 60 °C, which a parked car exceeds on a summer day. PETG buys you to around 70–80 °C, ABS and ASA to 90–105 °C, polycarbonate to 110–140 °C. Everything above that needs a heated chamber and a machine most people do not own.
Outdoor exposure. Ultraviolet light attacks some polymers and not others. PLA is the first to fail this filter: sunlight takes it chalky and brittle in a single season. ABS yellows and cracks because ultraviolet attacks its butadiene phase. ASA was formulated specifically to fix that, and PETG holds up well. If the part lives outside, this question eliminates more candidates than any other.
Flexibility. A real yes-or-no. If the part must bend and return, you are in TPU or TPE territory and the printing constraints change completely — slow speeds, a short constrained filament path, and a direct-drive extruder.
Stiffness against toughness. These pull in opposite directions and people conflate them. PLA is stiff and brittle; it resists bending and then shatters. PETG is slightly less stiff and considerably tougher; it deforms before it fails. For a clip, a bracket or anything that gets dropped, toughness wins. For a jig that must not deflect, stiffness does.
What you are willing to put up with. A material's difficulty rating is a real constraint, not a challenge. Unfilled nylon on an open-frame printer without a dry box will waste more filament in failures than the material costs.
What the printer rules out before the material does
Maximum nozzle temperature decides whether polycarbonate and filled nylon are even available. An enclosure decides whether ABS, ASA and polycarbonate are realistic at any useful size. A hardened nozzle decides whether filled materials are available today or after an order arrives.
Those three facts about your machine eliminate more options than every material property combined, which is why the selector asks about the printer as well as the part.
Where the honest answers live
Every material page on this site ends with what that material is genuinely bad at, because a shortlist that only lists strengths is a catalogue. The weaknesses are what make a shortlist into a decision.
If you already know your two candidates, the comparison pages go deeper on the specific pairs people choose between.
Two requirements that quietly change the answer
Food contact. No printed part is genuinely food-safe by default. The nozzle, the plate and the layer lines all introduce contamination paths that have nothing to do with whether the base polymer is food-grade. Treat this as a design question rather than a material question, and assume any printed part in food contact is single-use unless it has been sealed.
Gluing and painting. PETG resists most common adhesives, polypropylene resists nearly all of them, and PLA and ABS take glue and paint readily. If the part is one piece of an assembly that gets bonded, this is worth deciding before the material rather than after.
When the shortlist has one item
That usually means the temperature requirement is doing the eliminating, and it is worth double-checking the number before buying an expensive spool. "It gets warm" is not a specification; measure it if you can, because the difference between 55 °C and 75 °C is the difference between a cheap material and an enclosure.
When the shortlist is empty
Redesign before you go up the material ladder. A thicker section, a different orientation, a metal insert or splitting the part in two will often bring a requirement back inside a material you can actually print, and every step up that ladder costs money, time and reliability.