I have seen it time again and again people head out on the trail with either too much or too little gear. On one hand they weigh themselves down with unnecessary items and on the other hand they risk having the extra gear they need to spend the night. Below are a few points you should consider if you are deciding which to bring.
-The trail is long or the terrain difficult with even the remotest chance you will be forced to spend the night.
-You are carrying any gear.
-Your humping other people's stuff.
-Your cell phone won't work.
-You have young people and need additional safety items.
-You need a place to store limited items such as wallet, cell phone, bottle or water, etc....
-You have less than half a day's hike and you know you have access to shelter.
-Your cell phone works and you can call for help.
-You are only carrying for yourself.
-You need lightweight and limited capacity.
If you need a shoulder pack I will make a suggestion here. I have bought a couple of these and they work great for day hikes. There is enough room to put your keys, wallet, water bottle, and small safety kit. Furthermore, when I travel I have keep it with me so I don't need to load down my pockets. It is also harder for people to steel your wallet.
There is one left at $12.00 with free shipping in the U.S. It fits almost all size people because the straps are adjustable. This is new and never used. The one I have hasn't broken yet despite continued use.