It depends on what exactly it is you want to learn. My thought on RedHat is that you don't really learn much about the OS itself. It's a little too easy, as they want to give you little wizards and whatnot to separate you from what's really going on. This is not inherently bad (though there are other things about and surrounding RedHat that I think are), but it doesn't help you learn much beyond how to click the right buttons. It will allow you to familiarize yourself with it without too much knowledge to start with. I also have a problem in that it's become a standard of its own. It used to be that programs were written for Unix, and any Unix OS could run them. Then they started being written for Linux, with no consideration to the other Unices, and some now are being written for RedHat, ignoring other distros. But that's not really their fault, nor does it make RedHat inherently worse.

Mandrake I've not used, but, by all accounts (unless I'm thinking of something else), it's pretty much RedHat++. I don't think that you'll have much of a different learning experience with it.

The ultimate learning experience would be by using Linux from Scratch, but I've found that it can be a little tunnel-visioned. Still, you'll learn a lot, even if it's kinda got something of a bias.

Slackware would be somewhere inbetween as far as learning goes, but it's the oldest still existant distro, and, as such, has its own foibles that can make it seem disconnected from the rest of the Linux distros.

You can try Debian if you're interested in learning about open source politics. Many people prefer this one, actually, as it seems to be more for developers by developers. But that doesn't mean that it's either friendly or a good learning experience.

Those are about all that I have much familiarity with at all that are likely to fit what you want to do.

This fragmentation is one of the reasons that I find Linux hard to deal with these days. Each distro is almost different enough from each of the others for it to be considered a separate OS. Certainly, as a beginner, most of the stuff that you'll learn will be specific to the distro. Only with experience will you begin to understand where Linux leaves off and the specific distro begins. But trying to use a generic one may be too difficult initially and turn you off.