To mulch a slope, use a shredded mulch that's more apt to stay in place. Don't count on your mulch to change the pH of clay soil. If trees, shrubs and plants could vote on what covers their feet, it would be mulch every time. Not only do mulches dramatically reduce evaporation of water from the soil surface, but by reducing weeds they help prevent competition for water. As a result, soil moisture stays more constant and roots grow better.
Organic mulches save a bit more water than stone mulches, but just about all mulches do a good job.
And almost any soil cover is better than none at all. When the soil was mulched, only 1o percent of the water was lost from evaporation. Welsh recommends using a mulch that has both large and small pieces so that it will receive water well and also won't easily float or blow away. Use mulch on potted plants too. So mulch is good.