(this is way off the original topic, but to continue the segway...) There is an ongoing raging debate over the accuracy of smartphones vs wrist GPS devices. I think it boils down to personal preferences as to how we want to read and record our location data... wrist or phone.
In general: smartphone GPS mapping application software (depending on what phone, application, and version you might have) generally augments GPS position data with cellular location data, all of this with the intent of making your current location that much more accurate. However, it's hardly ever 100% perfect 100% of the time. At a minimum, let's consider some reality: we've got a very small piece of electronics that is communicating with satelites that are orbiting some 12,000+ miles above us, we are expecting it to be accurate within a tenth of a mile... AND we don't want to pay much for that luxury. We need to manage our expectations on accuracy.
I use an app on my phone simply because I'm already going to have it with me and I don't want another gadget to deal with.
Yes, sometimes the mileage is off due to spurious GPS spikes or signal bounce, and if it really bugs me that it's off, I can easily edit the data.
A running partner of mine swears by his Garmin, but I should share that his Garmin takes a long time to get a GPS signal lock and is less accurate than my smartphone.. Bottom line: if we want hard-core accuracy, measure out the desired distance and use a stopwatch...
my 2 cents... :-)
-bill