"My question was more about whether cell phones with GPS functions are as reliable as stand alone GPS."
Sort of, not quite. It depends. You have to define your terms. As a general rule, cell phones and smart phones can have a lot of stuff going on, and GPS location is only one item. (A stand alone GPS receiver has no such distractions.) Some smart phones have GPS applications running to do that positioning, and some have some downloaded map images. However, smart phones have a finite number of machine cycles, so things can get very sluggish very quickly.
As a general rule, a stand alone GPS receiver has a much better GPS antenna. This is especially true for the ones that appear to have an antenna sticking out of the top. The teeny tiny GPS receivers may have a tiny antenna that is about the same as what is in a smart phone. In a good location where you have a good view of the sky, the antenna doesn't matter that much. In a bad location, the antenna can help a lot. External antennas tend to be better yet, but they draw some power and cost some money. I doubt that there are many external GPS antennas made for a smart phone.
Smart phones tend to have a limited number of channels to receive on (the maximum number of GPS satellites to be received simultaneously). Again, in a good location, that is no big deal. As you get into difficult places, a stand alone 12-channel receiver can be quite handy, especially if you need maximum accuracy and fastest update.
Every smart phone and every stand alone GPS receiver must have an internal clock (oscillator). When the phone gets a connection to the network, the network clock can "tweak" the internal clock, and the internal clock is important in the early stages of trying to acquire satellite signals. However, if you are out in the boonies somewhere with no network signal, or if you have turned off the phone service, then you no longer get the benefit of clock tweaking. A stand alone GPS receiver tends to have a slightly better internal clock until it gets signal lock, and then the GPS satellite signals help it along, but it is quicker.
Along the lines of the network clock, there are accuracy differences from one carrier to another. Some carriers manage that very closely, and some let it slide slightly toward the outer edges of their network. That might show up as your displayed position jumping around more when you are standing still.
A stand alone GPS receiver will typically have performance statistics on that position error, so you can know when you can "take it to the bank" for accuracy.
Some non-techies do not want to fool around with all of the details.
So, it depends.