From what I've read, for the best outcomes the order of preference is:
- Identical sibling, e.g., twin brother or sister
- Perfectly matched sibling
- Perfectly matched unrelated donor
- Partially matched sibling
- Partially matched unrelated donor
but #3 vs. #4 probably depends on the particular circumstances, and in all cases the degree of mismatch is a factor.
I assume that sibling donors produce the best outcomes, all other things being equal, because they are more likely to match you in ways that the tests don't cover. But whatever the reason, researchers draw their conclusions by studying the statistics and factoring in age, the stage and seriousness of the disease, and other factors that affect the success of transplants.