Is there a difference between using a steel brush and a brass brush?
Also how hard do you press with either; a gentle caress or do you push harder?
As with most types of brushes, both steel and brass brushes come in many hardnesses and bristle-lengths, so the answer is "It depends." Some steel brushes are softer than brass, and vice versa. (Before the Sheldon Coopers out there jump on me, I'm speaking of the overall "feel" of the brush, not the actual hardness scale of the metal in each individual bristle.)
You need to guage your enthusiasm with the brush by what you are trying to accomplish - a little less elbow grease for removing softer waxes from the structure than for harder waxes, a little less agressive with a harder steel brush than a softer brass brush, etc. I like to use the brush for the job that calls for "firm pressure" but less than "aggressive pressure" to accomplish the job at hand.
Keep in mind that I am not speaking here about a super hard structure brush (like a very hard paint scraping brush) like the one I have from Swix (and probably others) that actually cuts structure into the ski; use those at your own risk!
This is the Holmenkol steel brush. The bristles are about 2 inches long and it is stiff and directional but much finer than some of the steel "barbeque" brushes. I use this with moderate pressure (one or two steady passes) before waxing to clear out the structure and with very hard waxes (regular brushing moderately firm multiple brushings) as the 1st or second brush.
This is the Holmenkol steel micro finish brush. I use it after stiff brass and the fine bronze if I am doing a serious finish for a big race. I follow up with horsehair and then a polishing synthetic brush.