This is really a pretty vague question and pointless thread...
For one thing, any Linux Distro can be configured appropriately to meet a number of needs. BUT it needs to be configured that way. And if you're serious about using Linux, you should be looking at configuring things, not at pre-packaged selections of software that Redhat or Mandrake things you shoudl install.
Further, you shouldn't be running a GUI on a fricking server... so if you are... stop it. You're hindering the machines perfomance and wasting disk space.
Also, if GUI speed is an issue, then the GUI is most probably the issue, NOT the LINUX DISTRO. There are MANY GUIs available for Linux Distros, however the default GUIs which install with many distros are seriously bloated and try to be too Windows XP super-friendly-ish ( IMHO ) Go look for another GUI ( other than Gnome / KDE ) and try that before you switch Distros.
Mandrake is not faster than Redhat, IF PROPERLY CONFIGURED.
Redhat is not faster than Mandrake, IF PROPERLY CONFIGURED.
Mandrake more than likely installs less stuff, so it takes up less disk space.
Oh, and if you're looking for a real workstation or server... take a look at FreeBSD :-)
Just my humble opinions of course...