I have two Gigabyte X99 boards and want to get rid of one. Not really sure which one to keep, so here we are. The X99P-SLI is about a year newer model wise.
X99P-SLI
https://www.gigabyte.com/us/Motherboard/GA-X99P-SLI-rev-10#ov
X99-UD4
https://www.gigabyte.com/us/Motherboard/GA-X99-UD4-rev-10#ov
X99P-SLI
https://www.gigabyte.com/us/Motherboard/GA-X99P-SLI-rev-10#ov
X99-UD4
https://www.gigabyte.com/us/Motherboard/GA-X99-UD4-rev-10#ov