I take your point. The screen size is larger on the PDA but it is portrait rather than landscape, that means that the text at the bottom of the map giving information like ETA, Mile To Go etc etc is actually smaller, likewise the popup touch screen keyboard for entering routes etc is much smaller (at least it is on mine). With the Go you get bigger buttons and the menu icons are larger too.
Perhaps you need to see them both side by side.
The beauty of the PDA is that you can buy proper car kits from Tom Tom and then fit the kit bracket as if it were a mobile phone. Here's a couple of pics of mine in my previous car to give you an idea. Selwonk also has a similar kit fitted in his which just uses a mobile phone mounting bracket.