Pocket Carry
I feel a pocket is a bad place for a spare mag.
Pockets get dirty and so does the mag.
The mag can change orientation in the pocket slowing a reload.
Pockets can be inaccessible, such as when seated, especially in a car, or while running.
You shouldn't carry anything else in that mag pocket.
Pockets are different on different pants: angle and orientation of top opening, width, depth, tightness which may lead to a slow or inconsistent draw.
If you are active, a loose mag in the pocket will tend to beat up your leg as it moves around.
If your pocket is tight and holds the mag securely in place, it is harder to get it out.
Available only to one hand.
If you carry the mag in a holster in the pocket, you have one more piece of gear to deal with on the draw.
Pocket carry may be convenient, but it may be less effective than other modes.
I prefer a IWB vertical offside mag holder positioned at 11 o'clock. Well concealed, quickly available. Always in the same position no matter what pants. Always oriented in the correct direction. Easy to remove with a C clip. Secure. Available to either hand.
As with all such decisions it comes down to personal priorities. Drop it in your pocket? Quick and easy. Draw it out fast, clean, correctly oriented, with either hand? Not so much.