Because of the AUR, Arch is more likely to have a package for some given obscure application that Debian would be missing. Also, these packages are kept up to date to a greater extent than you'll see on Debian. Finally they're all in one place where as you don't have to constantly add repositories to your package manager's repo list.
What you're mentioning are some of the main reasons why I am running Arch. But there's also the wiki, the community and the feeling of having a system which is very simple and clean. I tried it a few months ago and just loved it.
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