Materials
Glass and metal
The rigid, heavy, entirely non porous category. Nothing in this group wears out in any ordinary sense, which changes how it should be chosen.
Borosilicate glass and 316 stainless steel share the properties that matter: no porosity at all, complete indifference to any lubricant, tolerance of boiling water, and a surface that can be polished to a finish no polymer reaches. They also share weight and rigidity, which are either the point or the objection depending on the user.
Borosilicate is not window glass
Borosilicate is the glass used for laboratory beakers and oven dishes. Boron oxide in the mix lowers thermal expansion, which is why the material survives a sudden change in temperature that would crack ordinary soda lime glass. In an intimate product that matters, because warming or cooling under a tap is a normal part of use.
Glass in this form is far stronger than intuition suggests, but it is not unbreakable, and its failure mode is sudden. The practical rule is to inspect before each use for chips or star cracks, and to retire any piece that has one, whatever its condition elsewhere. A dropped piece that looks intact can still carry an internal flaw.
Steel grades
316 stainless, sometimes described as marine grade, contains molybdenum and resists corrosion better than the more common 304. For an object in regular contact with body fluids and rinsed in tap water, that difference shows over years as an absence of pitting. Polished steel also shows fingerprints immediately, which is only a cosmetic complaint.
Weight is the defining property of steel. A steel piece of modest size can weigh several times its silicone equivalent, and that weight is felt as pressure rather than as bulk. People who dislike steel usually dislike exactly that, and no amount of shape design changes it.
Temperature as a feature
Both materials hold temperature. A few minutes in warm water gives a piece that stays warm through use; a few minutes in cool water does the opposite. The safe range is narrow and easy to respect: comfortable to hold in a bare hand for ten seconds is comfortable everywhere else. Freezers and boiling water are for cleaning, not for temperature play, and a piece taken straight from either needs to be brought back to hand temperature first.
Cleaning and lifespan
Soap and water, boiling, or a dishwasher cycle all work. Neither material objects to any lubricant, which makes this the one category where a silicone lubricant is entirely uncontroversial. Barring physical damage, both materials will outlast the person who bought them, which is worth weighing against the higher outlay. The retirement page covers the small number of failures that do apply here, all of them mechanical rather than chemical.