The most important of all these uses for a back-country skier is to be able to dig in avalanche debris as fast as possible, so this should be the main criteria when considering buying a shovel.
Manuel Genswein, the Swiss guru of ski mountaineering equipment, recently made very thorough tests of snow shovels, in terms of strength and efficiency of use for the UIAA (International Climbing and Mountaineering Federation). His report is very in-depth but his general recommendations were:
Bigger is not better! When fast movement of snow is needed the speed of shovelling overrides the amount per scoop. Manuel Genswein found that a shovel size of 577cms2 (yes, he is very exact!) is the optimum size. This equates to a medium-sized shovel (cutting edge of 21.5 x 27cms deep). Large shovels with a flat blade are really great for digging snow pits to analyse the snow as they make a nice flat wall, but they are not good for the fast movement of snow as they can get stuck in the snow and each shovel load can be too heavy to sustain a rapid rate of digging.
Recommended shovels are shown below.
Genswein also found that the 'claw' method of digging (where the angle of the shovel can be altered) was not any more efficient than the normal digging method so having this function on your shovel is not a priority.
Shovels do need to be robust and Genswein tested all the current shovels on the market to destruction using 5 test methods. He established a UIAA standard which shovels should conform to. So, look for the UIAA 156 standard when you buy one. Shovels made of metal (generally aluminium) are strongly recommended.