What is the difference (Upgrade URL and snapshot Upgrade URL)

[kris_ini]

what is the difference between firmware
If a follow the link https://openwrt.org/toh/hwdata/asus/asus_rt-ac51u
can see two links for firmware

Firmware OpenWrt Upgrade URL:
http://downloads.openwrt.org/releases/18.06.1/targets/ramips/mt7620/openwrt-18.06.1-ramips-mt7620-rt-ac51u-squashfs-sysupgrade.bin
and
Firmware OpenWrt snapshot Upgrade URL:
http://downloads.openwrt.org/snapshots/targets/ramips/mt7620/openwrt-ramips-mt7620-rt-ac51u-squashfs-sysupgrade.bin

what to install, what's better?

[lleachii]

• The release versions (i.e. 15, 17, 18) should be used in a working environment.
• Snapshots DO NOT include the LuCI web GUI
• Once the Snaphot is updated on the Downloads site, you will not be able to install Kernel modules and other packages that depend on a the previous Snapshot's Kernel version

[tmomas]

See https://openwrt.org/releases/snapshot