You will find many discussions in the forums about presence detection using both phones and the Zigbee fob. Both work well for some people, both have problems for some people, some find one better than the other, but it varies which one.
There are a lot of things you can do to try and improve the situation. I ultimately ended up using two devices working together but it took several months of trial and error to get everything the way I wanted.
I am happy now. Presence detection was one of the things I wanted from smartthings to begin with, and now it is working. But it was sort of a twisty road to get here.