If you read the thread I link to above, you’ll see that it often just depends on local conditions. Boosted Wi-Fi, in particular, can cause the sensor to drop off.
I had an ongoing problem for a couple of months that support tried to help me with where every weekday afternoon around 330 My Presence sensor started doing the here/away/here/away/ pattern. I’m pretty sure one of my neighbors had boosted Wi-Fi and a kid who got home from school around that time. 
Anyway, there are various things you can try. Again they are detailed in that other thread.
In my case, I ended up adding a second device (these days it’s an IBeacon) to confirm that the away that it actually happened. Again, all detailed from the thread I already linked to.
So if you want to try to improve your use of the arrival sensor, there are various ways that you can. (I’m assuming you’ve already talked to support about it, because there are some settings they can change as well.)
But if that all seems like too much trouble and the phone presence is working well for you, I would just stick with that. 
As we discuss in the presence FAQ, there are multiple ways of tracking presence, and different ones will work at different houses.