Whichever you might like to use. Ubuntu is pretty vastly used other than other distro so that's why most spotlight goes to Ubuntu instead of Gentoo.
I don't see a hassle working with Gentoo if you already know how to use it more than Ubuntu but then again Ubuntu has less maintenance which is good for portable purpose. So I think you should go just like you suggested
Gentoo for desktop dual-boot with windows (xp/vista/7)
Ubuntu for laptop (also can dual-boot)
OR
if you have lots of free space to use on either desktop or laptop, just install both Gentoo and Ubuntu together on different partition. Much like below
Primary Partition C: - ntfs - Windows
Extended Partition - ext3/4 - Gentoo
Partition - ext3/4 - Ubuntu
Primary Partition - ntfs - Available free space for storage/backup purpose
Here's a note to take before installing any OS's on your computer.
* Note that Windows can't detect ext3/4 partition format so they will be mark as unknown partition in windows. Linux distro on the other side can detect all partition type.
* You have to install Windows first before install Linux distro because of boot process. If you install windows last, you may not be able to boot to your linux OS's.
* Primary Partition can be created up to 4 partition only while extended can break those limit. Be sure to set one primary partition to extended to extend your partition number.