My well rounded final choice is Samsung Galaxy Note 3.

1. It has removable battery - so you can stock on easily replaceable spares. And you can recharge them separately by various means while using the phone disconnected from the charger (must have option).

2. It has T-flash (Micro SD) slot. So you can carry multiple tiny memory cards with maps, music, books, movies, games, etc and make them available on the phone only when needed. I.o.w. you are not limited by the phone's internal storage (must have option)

3. Note has a decent stylus for hand writing, drawing, sketching, and precise manipulation of the graphical interfaceces elements. It replaces a pencil and a notepad in 90% of cases when those are needed (must have option to me, as I'm drawing plans and schematics a lot)

4. Has a decent camera (not the best possible, but very good, perfectly adequate for many daily survival purposes).

5. The screen is large, bright, and sharp - great for all mapping purposes, but still not big enough to make the phone as cumbersome as even 8" tablets.

6. Samsung is the king of various external sensors integrated.

7. OLED screen - great outdoors visibility even in the direct sun (consumes more power, yes, but you will have to take care of that in an ultimate way anyway) The OLED screen is a must have feature for me, as I need a completely black screen without any back light bleeding (All the TFT screens are suffering from) for the maximal darkness adaptation (serious astronomy, you know).

8. The OtterBox rugged cases are readily available for this top of the line model (as well as some other cases, including watertight and externally attaching battery options). A must have feature, as there are only 2-3 really trusted rugged external cases makers on the market to date. Also such wrap around cases are easily modifiable with a neck/hand lanyard and have a sturdy belt clip option.

The newer model, the Note 4 is also good, but it has a fingerprint sensor on the back, which makes it more vulnerable to damage and leakage (as the phone is often resting on some surface, plus - all rugged cases had to provide an opening in the back plate for that - another structurally weak point), but that's actually a minor drawback. The Note 5 and the upcoming 6 are no go, as they are sadly lacking the 1 and 2 features.

Regarding the power saving - the fastest way is to trigger the Airplane mode which is effectively blocking all of the communication chips and quite fast to switch back and forth. Also I made sure my desktop and apps have the darkest color themes possible (savings are for OLED screens only). Even my web browser has a night mode with inversed colors forcefully applied to any pages displayed. But if you carry a fully charged spare battery it's just a matter of 1 minute to replace it when low during the day. Under the heaviest use (watching HD movies nonstop over the WiFi with BT earphones) it goes from 100% to 5% in 4 hours. But I've been using it for GPS navigation including the constant track recording (also making surround images and short videos on the go) on an unfamiliar trail once for slightly over 8 hours in a row, and got around 30% power left on return.

And yep, the phone GPS is 100% functional without any networks around.