For a brief period, I considered using the open top style spare magazine holders but in the end, chose to keep using the covered style.
Dirt, crud, dust, lint, etc can eek it's way into your spare magazines.
Open styles may be faster but I prefer to protect the spare mag(s) from damage.
I've also noticed the sharp edges or metal in pistol magazines can rip clothes or tear skin. Ouch!
ClydeFrog