Circular saws have a number of different materials for both the shoe and the guard. The differences vary the weight, strength and cost of the guard and shoe.
Many budget saws use steel shoes which can bend if dropped. More expensive circular saws use aluminum or magnesium for the shoe. The magnesium shoes are light weight yet strong.
Some inexpensive saws use a plastic guard while others again use aluminum or magnesium. Look for saws with a magnesium shoe and guard if you want the best durability and weight savings in your next purchase.