If there's a long enough cord on the portable, he could hold it up high and directly in front of him, so that he doesn't have to take his eyes far off the road to operate it.

One of the things I liked about my '88 VW GTI was that its radio was mounted very high on the dash, so that I could operate it out of my peripheral vision without taking my eyes off the road. Now, in the Honda, looking down at the empeg (which, despite its clean UI, I still have to do sometimes), I have to take my eyes way too far off the road.

If I did a portable, I'd definitely use a cord long enough so that could lift it high up.