Well, considering that Slackware has a small manual with the CD set and Gentoo has an 80 page manual, I would say that Gentoo is a little harder to install.
Knowing only that I would say that Gentoo is more thoroughly documented. 
Having installed both though I do agree that it is a bit more work to get a gentoo system off the ground.
And garymax is correct; there is more to consider with a linux system than the first hour you spend on it. Gentoo's automatic dependency resolution is very good, and you'll appreciate it if you are likely to install software very often.
If this is just a server perhaps you won't care so much about gentoo's advantage. If it's for the laptop you mentioned in the openbsd topic then consider debian as well.
-kev
