Compare the features: the mix of USB/PCIe/SATA/Memory ports and their location, the audio section, network, fan headers and etc. If you can't find a compelling reason after looking at that then just look at the boards. If one's uglier than the other then avoid it.
Seriously: they'll probably perform very similarly.
That said: i got the Mortar. Something about that extra PCI-eX16/4 Gen 2 sitting there just looks good. Not that I'll ever use it, it just looks like it's a grown-up mobo.