I like the portable gps devices. If you have more than one vehicle you can switch easily. Plus the cost is much lower with the portable. I have a Garmin and I love it, I use it all the time even if I know how to get where I'm going and its shown me a couple quicker ways to get places.
I prefer the built-in when it comes to features, ease of use, the sound quality of the speaker, the larger screen size, etc.
However, there is something to be said for having a take-along GPS for when you get out and start walking, or go to another vehicle. However, on a feature-by-feature basis, I like it being built-in, but that's just me overall.
I prefer built-in because it is easier to use especially the ones with big screens. The Tom-Tom is cool though because you can even use it even if you are no longer driving.
The one drag about portable is I have to remove and hide it when I leave my truck. Leaving it stuck to your window is inviting a broken window and losing your gps. I like it being able to switch cars but being built in deters theft a little more, imo.