OpenWRT offers more customization / advanced features and is based off the linux 4.14 or 4.19 kernel. OpenWRT allows smart queue management QOS on the r7800 and dramatically lowers your bufferbloat (you can see this on the dslreports speedtest). OpenWRT on a r7800 typically will cap out at about ~850mbps.
Voxel’s build is based off the OEM netgear firmware (with a couple additional features). It offers full use of all the proprietary netgear hardware acceleration and NSS cpus (offers full 940mbps speed of all the netgear proprietary software ). r7800 actually has two ARM 1.7 ghz CPUs and two NSS 800 mhz CPUs... openwrt is trying to crack the code on these NSS CPUs but everything is close source / proprietary.
For the speed of your WAN line - the difference in speed between the two firmwares will be pretty much nothing. It more comes down to what features you prefer and if bufferbloat is a problem for you.