If you lower your Sensitivity you overall aim better. How? Its sort of like slow motion aiming and its sort of hard to explain it but your aiming is steady and accurate. The downside to it is that you can't move look around fast.
If you raise your Sensitivity you overall aim worse. I used to play at 100 Sensitivity and If i moved my mouse just a little it would go all over the place. The good thing is that you can do trickshots like 360's and you can look around more.
Result: 30 Sensitivity for Medium and CQC.
5-10 Sensitivity for picking off heads in Killcreek.
Remember this is my opinion and everyone has different sensitivity's.