I think that there are more differences between the phone/tablet & open source map solution and the GPS navigator & proprietary cartography solution than you have described.

I'm going to limit my comparison (below) to off-line use of phones or tablets (meaning, not having a continuous data connection via telephony), because that would be prohibitively expensive for international touring, and the need for a data connection would render the phone/tablet useless in areas not served by a 3G or 4G phone network.

The proprietary cartographic database used by dedicated GPS navigators is, on average, about 600 MB in size for either North America or Europe. That implies considerable road attribute detail and numerous waypoints (fuel, hotel, etc.).

I don't believe that the open source cartography contains the same road attribute detail as the proprietary cartography, in particular, details such as turn restrictions, time of day restrictions, and lane definition. If it did, we would be seeing the manufacturers of GPS navigators using open source cartography, rather than paying vendors such as Navteq or Nokia for their proprietary cartography. There's no easy way that I am aware of to compare the qualities of different cartographic databases, but comparing size of the database (for the same area) will provide a 'rough order of magnitude' comparison of the level of detail provided in each database.

There is no doubt that the speed of the hardware (processors) provided in phones and tablets has increased dramatically in recent years, and it is this increase in processor speed that has enabled map display and some navigational guidance to be provided (offline) using phones and tablets. But, at the same time, the manufacturers of the dedicated GPS navigators have also taken advantage of increased processor speed and capabilities to enhance the guidance their products provide. Examples of this include lane guidance, photo-realistic display of overhead signs, and spoken street names.

I think that a good analogy would be comparing cameras embedded in today's crop of phones and tablets to conventional cameras manufactured by companies such as Nikon, Canon, or Hasselblad. Phone cameras today take a remarkably good picture, and have come a long way from the fuzzy, marginally acceptable pictures of 10 years ago. But, if you want to take the best possible picture under the most demanding and varied conditions (this being the analogy to urban navigation), a 'dedicated' camera will give you better performance than a phone camera.

The other side of the coin is that if you just need a photo to identify something, or to show to a friend, or to email to someone, then there's really no point in going out and spending all the money to buy a dedicated camera.

Like I said in my first post, it's a question of "horses for courses". The task one wants to achieve should drive the decision of what navigation system to use. It's a fool's game to try and say one solution is better than another without first doing task analysis.

Michael