Depends how in the field you intend to be and for how long.
The sweet spot is probably a smaller 4Ah unit if size & weight is an issue. If you have a car nearby an adapter for that wins hands down.
Possibly if you have something that needs to work in a wilderness or always be ready as an emergency backup if there is a mains failure. A couple of thin mirrors will improve the solar panel output at the risk of cooking it. Mine (a freebie from some conference) has saved the day when our house mains went down with my mobile almost out of juice.
Weather and dust proofing is also a factor if it has to survive the rough and tumble of an outdoor wilderness existence. I have wrecked a mobile phone by fresh water immersion.
Depending on what you want it for there are endless choices. I now have one with solar panels that sits in a window most of the time and a smaller compact rectangular one that requires charging before use.