If you don't carry an extra mag, you should. Why? Magazines are the number 1 cause of weapons malfuntions.
Open top Kydex is the way to go.
Most people I know in the non .mil/LE world are best served by an open top Kydex mag pouch. Unless you plan on swimming through rivers, climbing peaks in the Takur Gar Mtn Range in A-stan, Wrestling with your local MS-13 gangbanger, flaps on the mag pouch don't make sense.
As for belt loop or paddle, that is a personal preference. You will have to try several on your own to make up your mind. I've used both and have a place for both.
Brands to purchase:
Bladetech - especially if you can get the model that holds 1 pistol mag and 1 flashlight. Frees up your pocked from carrying the flashlight.
Blackhawk CQC - single mag pouches that can be added to another piece of kit from BHI to make it a dual mag holder.