> I just bought all-clad stainless steel and don't have any cast
> iron, --is stainless steel just as good?

That's like asking "Which is better a hammer or a screwdriver?"
it depends on how you will be using it. A cooking method that
requires quick temperature response will want the All Clad with
its better conductivity. A cooking method that requires slow
temperature response will want the cast iron.

For example if you want to cut it into little pieces and do a
stir fry, not even the All Clad will respond fast enough and you
will want a thin wok.

You're question really can't be answered in general, only based
on individual preference. Some want non-stick, some want
steady heat, some want fast response. Each will prefer a
different tool.

> Also, where besides a supermarket would you buy steak?

For price-quality compromise I like Costco. For top quality find
a butcher and pay the extra price.