In very broad terms, steel can absorb more shock than carbon, but it's also less tuneable, so that it can wind up flexing where you don't want it to.

That's an incredibly broad statement and sure to be inaccurate for particular frames. Each material has a range and there's a lot of overlap. Very much of compliance (shock absorption, flexiness, power transfer, liveliness) depends on the frame construction - specific materials, the way they're put together, and geometry.

Very much of issues that are specific to you will depend on fit, too, and I'm still going to say that that's the #1 consideration. The most compliant frame in the world is still capable of transmitting shock in the wrong direction to a rider who doesn't fit on it. Your body has a great ability to absorb shock in the direction the joints naturally work. It's when fit is off and forces get transmitted across joints instead of through them where the muscles can do their job, that we wind up with problems.