feature and color
Mortar had:
2 extra RAM slot (4 vs 2)
64 GB RAM support (instead of 32)
2 PCIe lane [for GPU] (instead of 1)
more USB 3.1 ports (4+3 vs 2+4)
more audio ports (5+1 vs 3)
VGA port
and Xfire support (since it can support 2 GPU) [see the PCIe lane part above]