The steel-vs-plastic debate usually gets argued at the point of purchase, price, weight, looks, when the more useful comparison is what each material is actually like after a year of daily carrying, washing, and dropping. Here's what tends to hold up, and what doesn't.
Durability: the difference shows up gradually, not immediately
A new plastic bottle and a new steel bottle both function fine on day one. The gap opens up over months. Plastic is prone to microcracking with repeated flexing and temperature cycling, especially if it's ever been through a dishwasher or left in a hot car regularly. Once microcracks start, they tend to accelerate, and a bottle that looked fine in month six can develop leaks or cloudiness by month twelve.
Steel doesn't crack the same way. Its failure points are different: dents from drops, scratches to the exterior finish, and, as covered elsewhere, the lid and seal wearing out before the body does. A steel bottle body that's a few years old and dented still holds liquid and insulates the same way a new one does; a plastic bottle showing equivalent wear often doesn't.
Odor and taste retention
This is where the difference is most noticeable day-to-day, and it's usually what actually drives people to switch. Plastic is porous at a microscopic level and absorbs odors and flavors over repeated use, coffee, juice, anything acidic or strongly flavored tends to linger even after washing. Steel, particularly food-grade 18/8, is non-porous and doesn't hold onto smell or taste the same way, which is why a steel bottle used exclusively for water for years still tastes like water.
Temperature performance
A single-wall plastic bottle has essentially no insulation, cold water warms to room temperature within an hour or two depending on ambient heat. Double-wall insulated plastic exists and helps, but vacuum-insulated steel, particularly three-layer construction, meaningfully outperforms plastic at holding temperature over many hours, which matters most for anyone using a bottle across a full day rather than refilling frequently.
Where plastic still wins
Weight and price are real advantages, and worth naming honestly rather than glossing over. A plastic bottle is lighter to carry and cheaper to replace if lost or damaged, which matters for some use cases (running, cycling, situations where minimizing carried weight is the priority) more than long-term durability does.
The honest comparison
If a bottle is getting replaced every few months anyway (lost, given to a kid, used for a single trip), plastic's lower cost makes sense. If it's meant to be a daily-carry item for years, steel's advantages in durability, odor resistance, and temperature performance tend to outweigh the higher upfront cost and added weight, particularly once you account for how many plastic bottles get replaced over the same multi-year span a single steel bottle would cover.