A fixed-length shovel is lightweight and makes for easy storage in a backpack, but a shovel with an adjustable-length shaft gives you the potential for more leverage so you can move a lot of snow quickly. Some shafts feature extras like saws or probes stowed inside, to save space in your pack.
Handles come in three basic shapes: T-grip, D-grip, and L-shaped. The T-grip is the most common, and generally functions well. L-grips are the lightest in weight. D-grip handles are bulkier and heavier, but can be easier to hold, especially with mittens.
A plastic blade isn't strong enough to dig through concrete-hard avalanche debris. Aluminum is the only choice for safety in avalanche terrain. Some blades offer a serrated edge or pointed shape for extra digging power. Blade size is also a factor; those with larger blades will move more snow, but are harder to manage.