Techniques for selecting one of a plurality of heterogeneous memory units for placement of blocks of data (e.g., memory pages), based on both reliability and performance, are disclosed. A “cost” for each data block/memory unit combination is determined, based on the frequency of access of the data block, the latency of the memory unit, and, optionally, an architectural vulnerability factor (which represents the level of exposure of a particular memory data value to memory faults such as bit flips). A memory unit is selected for the data block for which the determined cost is the lowest, out of all memory units considered, and the data block is placed into that memory unit.