I am interested in purchasing a mobile device for my bike ride down the Pacific Coast to use in addition to bike maps. Can anyone give me an idea as to which device, such as an iPhone, etc. is best to use with apps such as GPX and others for riding?
As you are finding out the world of electronic navigation is a very specific and precise place.
Perhaps it would be better to think about, then describe just what exactly you would want this "device" to do for you?
I like to think of electronic navigation for bike touring in 2 parts;
Part 1 is the planning. Planning I like to see big areas so I can get the big picture of where I'm going. There are a variety of different online resources for that, and it is even possible to do it on paper maps and hen transfer that info onto the online maps or directly to the device.
Part 2 is following the preplanned route. That can be done with a smartphone (using many different apps) or a dedicated device such as Garmin, Wahoo etc. Each have their own advantages and disadvantages. Some have detailed maps, others not. Some work offline, others not. Some can deal with going off course, others not.
GPS devices for bikes are not like for cars where I can sit in my car and give it an address and off I go. A lot of dedicated apps and devices can do that.... but the quality of the route they pick is often questionable.
So, come back and tell us what you want this device to do and we can be more helpful