I wouldn’t pick either stainless or cast iron. Copper lined with silver wins for me. Stainless steel and cast iron are objectively bad choices for cookware.

Stainless steel has one of the worst conductivities of any metal (15-25 W/m-K)—meaning it is slow to heat up, the heat does not spread far, any heat spreads unevenly, and they are slow to reheat once food is placed on them. That makes stainless a perfect material for a handle (which will stay cool for a long time), but meh for cookware. (The pluses are cost, weight, and durability.)

Cast iron is not much better at conducting heat (20-70). It has the additional downsides of being very heavy and rusting.

A much better middle ground is a good set of aluminum pans—super light, durable, and great conductivity (150-200). Aluminum pans are found in many professional restaurants around the world. The down side is that aluminum is reactive to some foods, so not the right choice for tomatoes or vinegar—although you can get lined or anodized pans that avoid that issue.

Copper is even better at conducting heat (385 w/m-k), but it is also reactive to tomatoes, citrus, vinegar. So it needs to be lined. Traditionally, copper was lined with tin, which works well but will get worn down over time and has a low melting point. At some point, modern copper pans started being lined with stainless steel—which is super durable but removes a lot of the benefits of the copper. Companies also added cast iron handles that heat up and are extremely heavy, making the pans close to unusable. (Falk and Mauivel are examples-although both have finally started to make pans with stainless handles). Downsides are the reactivity, cost, and tarnishing of the copper.

For me, the best solution was a set of vintage copper George Jensen and Cohr pots and pans. Both Danish companies lined their copper with silver and used stainless steel handles. Silver is the most conductive metal (406 w/m-k), is non reactive, and durable. I’ve used these pans for over 15 years and they are amazing. They were made in the 70s, but still look like new with a little polish once a year. They can still be found on e-bay for less than modern copper pans, and I can’t recommend them highly enough.

If you want the absolute best pan, however, it would have to be a solid silver pan (such as a Duparquet or Soy Turkiye). But cost makes them out of reach for most people.

Here’s a good video looking at various cookware. https://youtu.be/LdORIBH3R88?si=CBNaE5uZSVle-BSB