Hi,
At the console level there is not a great deal of difference. Mandrake is based on redhat and is therefore highly compatible with redhat. There are one or two differences - for example, the way Mandrake package the Apache webserver is rather different to redhat.
The main differences, however, are in the gui tools that are provided by each distro and in the speed with which they get new versions and updated rpms out. Mandrake seems to be edgeing it on both of these at the moment.
The other issue is support. Mandrake has very good web support but is not as widely used as redhat so, overall, there are more people who can help with redhat than the other distros.
Its really a matter of personal choice - Mandrake gets excellent reviews for its' ease of use for newbies but, personally, I've never been tempted away from redhat as my main distro.
Regards