As a former Arch user, the biggest difference for me is:
Ubuntu is a serious distribution. It's the most widely used both on the desktop, in servers and on the cloud. No other distro has more packages in their repos than Debian/Ubuntu and then you have PPAs if you want even more. Even pacman + the AUR is inferior and keep in mind that the AUR is a second-class citizen, you have to deal with orphans, flagged (outdated) packages, etc...and it's not officialy supported by the Arch developers. You have to use it at your own risk. The same applies to PPAs but you will rarely need to add some because the repos probably already contain what you will need. And PPAs can be easily updated with "apt update" while for the AUR you will have to use an AUR helper, which brings more problems and also isn't supported by the Arch developers...Ubuntu guarantees 5 years of support if you stick to LTS and every ~ 6 months you will get updated kernel and graphics stack. And if you want the newest kernel at the moment it's released it's easy to upgrade thanks to kernel.ubuntu.com, if you want to be more on the bleeding edge you can use the current regular release and enable pre-released updates. But Arch is more bleeding edge no matter what you do with Ubuntu, it's in his very nature. Even Debian Unstable is old compared to Arch. BUT people in Canonical get paid full-time wages to support your system while Arch is a hobby. If the developer is busy with his life you are fcked.
Arch is a community distribution. It's for fans and enthusiasts but it's not a distro one will choose if he has serious work to do. Pacman is great and the rolling-release is great too, you will run the newest software (if it's in the repos because they are much smaller than Ubuntu's) but it's more demanding. You can't just concentrate on your work because the system itself becomes part of your work. This is why Linus himself prefers 'easy' distributions like Fedora, openSUSE and Ubuntu (he dislikes Debian, but likes Ubuntu because 'they made Debian useful' - those are his words, google it).