JFS is very soft on cpu usage, and ext4 does a very good job overall. fsck times for ext4 makes it probably the best choice for a server, plus it has more eyes watching over it.
In any case, I wanted to call your attention that this might not be the best choice anyway. If you truly want portage to have the minimum possible impact on the performance of your machine(s) you should probably be using other machine(s) to build binary packages, then use these package in the target system(s) (i.e. with emerge -K). -- Jesús Guerrero Botella
