ok here is my take on this
gentoo is harder and more complex to isntall as there is no installer and you put it all together my hand but they have a wonderful installation manual that walks you thru the whole process and then setting up X is alittle more difficult as you have to configure it yourself but again the manual walks you thru this

and once you have a fully functional gentoo system it is really easy to keep uptodate with the portage system that is built into gento. it is wonderful and makes installing and uninstalling stuff easy
slackware has an installer so it is alot easier to install and set up it gets everything running for you but it isnt as easy as say fedora is. but still pretty easy to work with. once you have a working system you dont have the one command install thing with slackware like you do with gentoo. but there are a bunch of 3rd party apps that allow for this kind of thing like swaret witch is like apt-get for debian or yum for fedora. and then there is another (cant think of its name right now) which is like the portage system for gentoo, actually it is a port of the portage system that gets everything from the same mirrors that the portage system for gentoo does
i have always wondered do you think if i used the portage port for slackware and did an emerge world i would change my system from slackware to gentoo? just something i was wondering
edit: i am currently using slackware but have ran both and decided i like slackware better cause im to lazy to wait to get gentoo running cause it will take you some time to do not just a install in 2 hours type of thing like most distros
