As a former stand builder and currently an industrial painter I'm obviously biased
I feel strongly that steel will last virtually forever if prepared properly (I warranty any stands I have built in the past and will continue to cover them for many more years.
The "powder paint" you are referring to is called powder coating and is indeed a very durable coating. Personally I'm not a huge fan of it as it isn't indestructible as people claim and once you notice rust, it's likely pretty far gone as it travels and grows beneath the coating. We remove a lot of old powder coat at work and 9 times out of 10 rust spots on the surface indicate a badly pitted surface underneath. Also refinishing powder coat is a pain as it can't be touched up, but rather removed entirely (we use abrasive blasting). Cost wise, it can be even cheaper than a quality paint job as its a pretty competitive market right now.
If you decide on a steel stand, look for details like closed (mitred) corners instead of open tube ends (impossible to get a decent coating inside and rust traps). Also check the build quality in terms of design. I've seen store sold stands where the weight of the tank is supported only by a half inch weld. Check that the weight is transferred from the top through the uprights (hope that makes sense).
That being said, a quality wood stand should last as well and is easier to match your decor.
Francis