Hi GG:
This is mainly a question of preference. Many old-time assayers swear by bone ash cupels, and they do have the advantage of working with a slightly lower temperature (maybe 50 degres F less) than MgO. But they have the disadvantage of being softer, much more expensive, and more fragile.
When we used Mabor MgO cupels for bullion analysis, we had excellent results (never lost an umpire). This may be due to greater consistency (even if the actual loss is higher).
But, if you prefer bone ash, and have better results, and can stand the price...
More information on both types here:
http://www.Lmine.com/go.mvc?id=cupels