both can support SATA mode or PCI-E mode.
Depends what sort of board you are looking for.
I think this table will be perfect for you , if you need to buy X99 motherboard.
https://forums.overclockers.co.uk/showthread.php?t=18740078
focus on PCI-E mode. such as PCI-E X4, X2...... speed is different.
I don't think so sata mode is important. because you buy X99 motherboard you will choose SATA mode M.2?
I think you will choose PCI-E M.2.