What's the difference between somebody with hemophilia a and someone with hemophilia b?

Different factors: The difference is in the clotting factor missing. Hemophilia a is caused by lack of the blood clotting factor viii. Hemophilia b is caused by a lack of blood clotting factor ix. They both lead to the inability of blood to clot properly, just in different ways.

Deficiency factor 8.: The difference is that hemophilia a is due to a deficiency of factor 8 (viii), while hemophilia b is from a deficiency of factor 9 (ix). Both are sex-linked recessive, and both have similar lab findings of a normal pt and high ptt. The treatment of each is replacement of the respective factors. Hemopnilia a gets factor 8 while hemophilia b get factor 9 in concentrates or fresh frozen plasma.