I don't have much experience with the .40, but with the 9mm, contrary to what you might think, it is actually the heavier bullets that have less felt recoil.

The lighter bullets with their higher velocity have a more snappy recoil, while heavier bullets give more of a "push" than a snap, which some people find easier to shoot.

All .40's feel pretty snappy to me though. And I would think as long as you have good hits, any .40 should be enough to take down the steel, if the targets are calibrated properly.