I think everybody argee GPS is very useful and popular now a day. (And I cannot live without it for long trip)
Of course it would be nice to have both. But if I have to choose, I rather give up build-in GPS because adding GPS functionality is very easy by BT but not USB host.
For me, I use my Eten M600 as GPS and MP3/Video player in car. By using USB host w/ an external HD, I can store more media files for the long trip. Maybe swapping unuse map to HD too.
My SE X1 rom-> http://forum.xda-developers.com/forumdisplay.php?f=471