The goal of this clinical trial is to compare outcomes of two reduced intensity conditioning (RIC) regimens (fludarabine plus busulfan and fludarabine plus melphalan) in allogeneic hematopoietic stem cell transplantation (HSCT) for acute myeloid leukemia (AML) or myelodysplastic syndrome (MDS) patients. The main questions it aims to answer are: - The safety of two reduced intensity conditioning (RIC) regimens (fludarabine plus busulfan and fludarabine plus melphalan) in allogeneic hematopoietic stem cell transplantation for adult AML/MDS patients with HCT-CI≥3 or aged ≥55 years. - The efficacy of two reduced intensity conditioning (RIC) regimens (fludarabine plus busulfan and fludarabine plus melphalan) in allogeneic hematopoietic stem cell transplantation for adult AML/MDS patients with HCT-CI≥3 or aged ≥55 years. Participants will be randomized to one of two reduced intensity conditioning (RIC) regimens (fludarabine plus busulfan and fludarabine plus melphalan)