Let's go through the different aspects of phone versus dedicated GPS device point by point (however, quite surely without being complete). Basically you have to decide which points apply in your situation and how you weigh them.
GPS reception and accuracy
When the first smartphones with GPS units hit the market, there was the saying that their GPS positioning was less precise than that of dedicated GPS devices. The technical rationale given for this is that the GPS antenna of a smartphone has to be mounted hidden somewhere within the phone's body where it might not have optimal reception. In contrast dedicated devices have an antenna that is positioned in an optimal way, even if that means that the antenna sticks out a bit. However, that does not mean that the GPS accuracy in a smartphone is necessarily worse than a dedicated one, as for example this test has shown. State-of-the-art smartphones, especially the higher end ones, are in principle able to play in the same league with dedicated devices.
But there are two possible drawbacks due to the internal antenna:
The reception depends more strongly on the orientation of the device with respect to the sky, i.e. accuracy might worsen a bit if you just throw your phone into your backpack and it rests there in a way that the GPS antenna is shielded towards the sky by all the electronics, the battery and parts of the case. When held in hand, there should not necessarily be a difference.
Basically the same limitation occurs if the device is used in conditions where the sky is to some extent shielded by the environment – imagine being in a thick forest, maybe additionally in rainy weather with all the trees full of wet leaves or in a deep valley with not much open sky above you.
Battery life and endurance
Here the dedicated device is clearly in favour, however, it depends on the use case, how big this advantage is.
If you use the device for full navigation and/or tracking, i.e. doing location updates and maybe having the display on all the time, your smartphone might easily only last for some hours, even with all unnecessary things (WiFi, Cell Connection etc.) turned off. Even if you just use your phone only to look up the location every now and then and keep GPS off the rest of the time, it might suck a lot of power just by having to run its operating system. There are lots of processes running on the phone all the time that will drain your battery even in standby. Turning the phone off when not needed would be impractical as well, because it will take some minutes to boot until it is ready to use and booting a full-blown smartphone can easily drain several percents of battery.
A dedicated device on the other hand has a very slim operating system that is not using much battery when idle and also does boot up quickly if you prefer to shut the device down if not needed. Above that, there are devices that run on standard AA or AAA batteries which makes it easy to bring some sets of spare batteries. Those give you quite surely more battery lifetime per weight for your dedicated device than a battery pack for smartphones and you can tailor the amount of batteries to take with you depending on the length of your trip.
Finally there's also some safety issue if you carry your phone also for emergency contact reasons: by draining your phones battery with GPS use you do not only lose your navigation but also your phone functionality.
If you want to have maps on your device, your mileage will vary with both devices. With a phone you can basically have every map that is available as long as you have cell coverage and many maps to be downloaded to the device if you have the right app installed. With a dedicated device it depends on what the manufacturer allows you to install on it and how easy it is to get additional maps for certain purposes like sailing, hunting or the like. This depends also strongly on the manufacturer or device.
Handling issues can be divided into software and hardware based ones:
On a dedicated GPS you are basically limited to the software that the manufacturer has designed for the device and the functionality they give you. Hence it is important to have a look at your need and the provided functionality to chose a device that suits your needs.
With a phone on the other hand, you can basically get apps for everything, but not necessarily one that will bring all you need. Also as far as I see, there's some divide into small, slick apps that do only one or two certain tasks on the one hand and big, heavy ones at the other hand that provide a lot of functionality (which is mostly not needed for your use case), at the cost of higher battery drain.
On the hardware side I see a clear advantage on the dedicated device for several reasons:
- Most of them are more or less waterproof which is not only a matter of carrying them in humid weather or for tours around water but also for example when having them get moist in the pocket of your hardshell jacket or trousers on a sweaty ascent (yes, I know of people who have killed their smartphone that way). Also using them with dirty hands is no problem.
- They are constructed for harsh environments with a rubber casing so they won't be damaged seriously if they get some scratches or fall out of your hand or pockets.
- Most of them are designed to be usable with gloves, so no need to get your gloves off. This can be essential in cold winters but is also nice on bike trips where it's quite annoying to get off your gloves just to see how far you've already ridden.
- Finally, and for me one of the most striking parts: have you ever tried to read something on your smartphone display on a summer noon in bright sunlight? You won't have any chance since the background illumination of the phone display is not strong enough to outshine the sunlight. Most GPS devices have instead transflective displays that use the ambient light instead of a backlight and stay readable in the brightest sunlight. They have a backlight as well, however this is only needed for low light situations and thus saves you battery time.
Basically you have to decide yourself and I hope to have given you the most important properties to influence your decision. If the device is somehow part of your survival plan (this is what your question sounds like) then I for myself would go for a dedicated device since the smartphone solution appears to have many more modes of unwanted and unexpected failure.