I think both are important in global food supply and in terms of posing challenge on feeding humanity. Food Waste is more prevalent in the developed world where surplus production is possible. Food Losses are more prevalent in the developing world where there is limited technological capacities to process and preserve agricultural commodities and where production of bulk foods with consistent properties is challenging for the development of food industries.
FAO generated a report on Food Waste and Food Losses for different regions of the world in May 2011. You may access the full report following this link.
They are both of significance when it comes to the availability of food especially when it relates to developing and developed countries. In developing nations food loss as a result of post harvest losses account for a significant reduction in the availability of food. In the developed world food may be wasted at the consumer end . So to answer the question......it may be that food loss which is the primary factor for losses in food in already food insecure nations such as developing countries around the world just may be more critical.
It does not matter whether it is food loss or food waste since both result in food that could be consumed now not being available (especially for the hungry masses). I will venture beyond norms and say that wasting food should be considered an offense and people should be held accountable and called out when they engage in such activities.