Tom Ritchey We all bought aluminum instead of steel for awhile because it was radical and lighter in weight that our old steel bikes. After a few years of paying more money to get beat up, we remember why steel was used in the first place. You are relying on the frame for shock and absorption. Aluminum frames make sense for full suspension bikes because the shock system soaks up the bumps. The stiff tubing will fight flexing pivots.
Some companies specialize in steel and build frames that are very light-weight.
Steel frames are built for medium to long rides. Aluminum frames are popular for races, which usually last for an hour. The bike brands they carry don't offer it. The salesperson is not being realistic about your needs. Compare similar bike ranges and remember, put your money in the components and let the steel take the bumps.