Depends on what you want to learn...
For those distros that you've mentioned I know that you are guaranteed to learn one thing very well... How to use a mouse and click on things on the screen. Other than that, I'm not so sure.
Paying is good if your intention is to contribute to the company/person producing the distros... just remember that some are more worthy than others.
If you haven't used Linux before, I'd suggest something light
but still Linux-like... say Mandrake. If you are familiar with Linux but would like to learn more administration skills, then probably Slackware and/or Debian. If you really want to learn Linux inside and out and not afraid of the command line, compiling, debugging, reading tons of docs then probably you'd appreciate something like LFS.
Pick your poison