The NAV in the Tech is nothing to write home about. Get a standalone GPS with cool 3d graphics and be done with it… I have a Tech and I never use the NAV due to the outdated graphics and maps, I also live in México so the full North American maps in my TomTom GO600 have been a life saver..
I often use my built in Navigation as its much more easier to follow than small screen of smart phone, also its well integrated with audio so I dont miss instructions while listening to songs irrespective of the source. The only downside is outdated maps and lack of traffic.
I used phone only when I need traffic based routing. Now with AA with Navtool its definitely much more convenient.
However For a base MDX I would also recommend buying a standalone GPS, some of these come with traffic routing as well so those will be somewhat at par without needing Data.