Go with the Nav. I was skeptical but glad I got it. Here's some advantages I've found so far.
1. An 8-inch screen
2. You can control everything using voice commands without taking your hands off the wheel
3. Turn list view. Because of the big screen, MFT Nav can display the map and the turn list at the same time. It's so helpful knowing the next couple of turns
4. Music volume is automatically lowered when MFT Nav speaks
5. Dead-reckoning. MFT Nav is integrated with the car. When there's no GPS signal(in a garage, tunnel or under tall buildings), it estimates the location using gyroscope, vehicle speed, steering direction and etc
6. Next turn is displayed on the 5'' LCD in the instrument panel. This helps keep your eyes on the road
7. Route recalculation is almost instant. As soon as you miss a turn, it guides you to the next turn