When comparing pikaur vs yay, the Slant community recommends yay for most people. In the question“What are the best AUR helpers for Arch-based Linux distributions?” yay is ranked 1st while pikaur is ranked 3rd. The most important reason people chose yay is:
Yay's commands and output make sense for anyone used to the pacman package manager.
Ranked in these QuestionsQuestion Ranking
Pros

Pro AUR package names in shell completion (bash, fish, zsh)

Pro Upgrade -git, -svn and other dev packages

Pro Using systemd dynamic users if building packages as root user

Pro Show unread Arch news before sysupgrade

Pro Interactively handle common build problems (like untrusted GPG key or checksum mismatch, wrong architecture)
Pro Can install packages even when others fail
Sometimes when building multiple unrelated packages, the failure of one means that none get installed. With this helper, it will not only ask you what to do during a failure, but you can skip the package all together without having to restart.
Pro Remove make dependencies on completion
Some AUR packages require at times dozens of dependencies solely for the build process. Usually, once the build process is done, they stick around without purpose. This helper automatically removes those dependencies once all the builds are complete.

Pro Retrieve PKGBUILDs from AUR and ABS (-G/--getpkgbuild)

Pro Build local PKGBUILDs with AUR deps (-P/--pkgbuild)
Pro Intuitive CLI
Yay's commands and output make sense for anyone used to the pacman package manager.
Pro Written in Go
The compiled program is snappy while the source is easy to read.
Pro Available as a precompiled binary
Both yay and yay-bin are in the AUR, the latter of which doesn't require any dependencies or compilation, making installation and updates quick and painless.
Pro Yogurt interactive mode
Write package name without keys [yay <packagename>] to enter interactive mode.
Pro Easy to add features
It's written in Go so it is fairly easy to add features or tweak this amazing tool.
Pro Doesn't rebuild already-installed apps like Trizen
Cons

Con Splits pacman -Syu to -Sy and -Su
-Sy (to refresh package list first) and -Su (to install upgrades after user confirmed the package list or has been altered it via [M]anual package selection).
