I didn't do any "testing", but whenever I'm out geocaching, I use my Samsung Galaxy Mega simply for ease of use and on the spot logging. I always carry my GPS unit, which is an ETrex Venture HC, because sometimes the phone option won't work. As long as I'm in the open or light tree coverage, the phone works great. If I get into heavy overhead coverage, it starts to lose its accuracy. My GPS will work in the heavy coverage where the phone starts to fail. The phone also has another drawback, which is that if you get into a place where there is no internet coverage, the maps won't update if you try and zoom, move them around, etc. I like having the maps available, so my GPS unit works in those places.
As for accuracy, in the open I found no difference between the two. Recently, I relocated a cache for someone, and I got curious if my phone would be accurate enough to get the coordinates to send in. Both my phone and GPS showed the exact same coordinates. Earlier in the day, under some dense tree coverage, I had an 18 yard difference in my location between the two, and it was the phone that was off.
A bit off topic, but useful info for accuracy when you start thinking about signal strengths. GPS still wins.
