I've been using Ubuntu for about 2 years now. I've learned how to use it pretty well. I know how to use the command line and I've learned basic shell scripting and stuff like that. I would consider myself on the lower end of intermediate.

I want to try a more advanced distro for the sake of learning how Linux works, not just how to use it. I'm thinking either Arch or Slackware. Can anyone recommend either of these or something else? Also, I plan on running it in VMware so any info on that would also be useful.

I really don't want to use Gentoo unless somebody can give me a good reason why. Also, Linux From Scratch is probably out of my league right now.

Slackware (for 32-bit systems) or Debian will be most suitable for your purpose.

I have spent time with both Arch and Slackware, and I now favor Arch. It's a very close call---for me, the more straightforward package management in Arch is the deciding factor.

Slackware is pretty nice. The Slackbook that they publish on their website is great. It helps you configure it properly while at the same time allowing you to learn about where config files are, what the settings mean in them, etc. slackware is not easy to update however (at least not when I used it). In order to update, you sign in to their patch e-mail network and when a patch is available, you manually download it and install it. No out of the box automation. I have heard of some gui tools for this but haven't used them.

mmm debian, but if your looking for something that people tend to use in business I would look at centOS or fedora.

Crux linux is also another good distro for learning how it works. Not much hand holding with it.

http://crux.nu/

You can use Slackpkg for updating. Easy and reliable.