I've recently installed both Gentoo 1.4 and Mandrake 9.0 (both bleeding edge distros). I'm not going to talk about the installation so much as the niceness factor. The niceness factor in my opinion is after doing a default install, how nice is it to use. Gentoo 1.4 hasn't been released yet, so things may change.
My computer has a removeable hard disk, so the configuration was the same for both systems.
It should be noted for those who aren't familiar with the two systems, Gentoo is a source based system. You compile the whole system on your computer. It takes time. You can install precompiled, but that removes what makes Gentoo special. Mandrake installs precompiled code. Gentoo uses its own system call portage to do updates. Mandrake uses urpmi.
Mandrake is easy. Glitches... a few. It hung while looking for printers, and final updates at the end of the installation. A reboot, and selecting update instead of install, quickly caught up to the last step accomplished, and things continued without problems. (I avoided getting final updates). It did correctly detect and test the local printer, (something I just haven't gotten around to with Gentoo). (it made me happy) If you are the type of person who like to click and go, this one is for you.
Gentoo not so easy. The thing about Gentoo is that it is made for people who like to tinker with their system. When you install it you are given step by step instructions and you learn as you are doing it. Recompiling the world also takes time, and once things are configured your computer will churn for quite a few hours, compiling all the code. The Gentoo install is not so easy, but it's better for the experimenting hacker. Interestingly, I didn't have any glitches with Gentoo, even though I had to do more work to get things going. The documentation is very clear and if you are the type of person who builds models or likes kits this one is for you.
I'm a glutton for software. I always install everything I see that looks neat. (all of it). Both distros are gcc 3.2 which seems to cause grief to C++ programs. It looks like both distros have done a good job of getting things working, with this major change. My system uses the SIS chipset which although claims to support Linux, really doesn't do a very good job of it. In the past sound and 3d graphics have always been a headache. The Mandrake did a better job of configuring the sound, and was able to get 3d working straight out of the install. As I had to figure this out manually with Gentoo I was impressed with this. Gentoo, relies on the users ability to problem solve in the area, so it can be as good as it can get, but it's up to you... I'm not sure that Mandrake's opengl is as good a setup as the Gentoo one, that I configured, it seems slower. But the fact that it did it, suggests to me, how much they are doing to make the hardware a smooth installation. (I think I'm going to peruse the config files to see where the two differ:) Both Distros include Gnome 2.02, and KDE 3.03. Both Distros have all the software that is worth having available to them, with only minor exceptions. (I like festival, but it won't compile under 3.2 for some reason. A problem being worked on though).
You can update software with both systems. And this is where I feel the most significant difference lies. Mandrake will allow you to click a button and a little box pops up, saying "searching for packages" and that's it. (Something is seriously wrong there). I suspect it might be that the mirrors were all overloaded with everyone downloading new ISO's but even if that were the case, a little feedback would go a long way to ease frustration. Gentoo uses a command line utility called emerge. emerge will go to a site and download the source and then go through all the steps necessary to compile it. What I like about this system, is it tells you what it's doing at every step of the way. Downloading info, unpacking info, compiling info, all the compile messages, any extra tips needed, and any configuration files that you will need to look at. (It won't change your configuration, but it will put what it thinks is a good configuration in the same directory, with a different name).
The niceness factor... both are very nice. I use the KDM to allow multiple logins (my family suffer through me updating the linux box) I have many different windows managers. Fluxbox is so fast, it's like it was already loaded, KDE is so feature rich it makes commercial systems seem irrelevent. I use Gnome as well, and have no complaints about it either. There are others available to my wife and kids, but those are the ones that I use. Gentoo, is noticably faster. It makes sense because you configure the compiler to be optimized for your system in Gentoo, whereas Mandrake must make a general system that will handle the lowest common denominator. Gentoo also has a slightly larger package selection available. Also the Mandrake Club is quickly catching up with volunter RPM builds.
I'm going to use both. I'm going to mix and match the features until I've got the features that I like the best."