For easy reading, I recommend getting one with a 4.3" to 4.5" screen & a 5" if budget isn't a major factor - whether it's a standalone like our semi-retired Garmin or Google Map/Nav-based smartphone. Bigger screen made it easier on the eyes especially while driving & more space to input criteria if voice search isn't available. Newer Garmin & TomTom, etc. now often come with free ifetime map updates, except for its base/no-frill units to stay somewhat competitive as Sat NAV - as frequent updates are very helpful while visiting new & unfamiliar places.

For backup/offline NAV, I have been using MapFactor Navigator on our Moto Atrix 4G (free OSM data version with worldwide maps, by country & often subregion with free data & frequent updates as released to stay current.) It is available for Europe, Asia & of course, the Americas. Unlike the latest Google Map/Nav with 3D views, lane assist and other advanced features only available recently on high-end Sat Nav standalone devices, and Waze integrated for real-time traffic, a blessing in busy urban areas like SFO, LAX and metro NY, the Navigator is great on not using data and have decent directions - and can be loaded with optional (free) Speed Camera data (red lights & speed limit types) & warn if exceeding the posted speed by 5% or 10%, etc.

We're had a long road trip to/from the Grand Canyon, AZ in late October where I knew/expected to have spotty AT&T and T-Mobile data coverage, and sure enough, the Navigator with preloaded AZ maps kept us right on course, even inside the national park & steer us from our lodging to various scene stops and out of the park driving out to LAX - for about 3 hours, I only used its offline NAV & never got lost on our planned routes with stops, etc. Our Moto has a 4" screen only, was adequate but I absolutely loved using the regular Nexus 5 with nearly 5" screen, voice search & with live data, plotting our best way all around Los Angeles during our 5 days in town.

I have tried out a # of free smartphone mapping apps and like the Navigator since it's already on my backup device wherever I go, always available. The data is stored on a 16GB mSD card and even when I reinstalled the ROM, everything was good (both factory & custom Roms) It will store my search results ahead of time under "My Places" so for quick access - using Google, POI and Coordinates. FYI - the paid Navigator version runs on updated TomTom maps.

For mounting to rental cars, I have the $4 vent clip in my gadget pouch and it's been great, on road trips for about 20 days since mid 2014, and adapted to various car vents easily - and smartphone never once slipped out from bouncing & bumping road or on turns, etc. (AND, yes - make sure you have a high amp USB car charger as it's a battery killer ...)