I want a double mag holder for the obvious reason of more ammunition - but when sitting in a car for an hour or two - this becomes quite uncomfortable, which is why I'm thinking about a single mag holder.
I am becoming a fan of Fobus, their products have a very nice tactical look to them and their prices seem great for the quality of their products.
I have the HP2 holster and it fits my gun like a glove. I plan on picking up thier single magazine holder just because I can't really imagine a situation where I would need to use more than 16 rounds.
When carrying on the street, I wear a single mag carrier from RM Holsters. When training, I wear a second single mag carrier behind the first, and use it to feed the primary (front) carrier. I don't train to draw a new mag from the rear carrier, because I normally wear my cellphone in that location and I don't want to jack up my muscle memory and end up loading my Samsung into my Glock if TSHTF. You laugh, but I saw a friend of mine do that with his Leatherman once...
I also prefer wearing two singles to a double carrier, b/c they allow greater space between the mags, which enables a cleaner grip and drawstroke, IMO.
Fobus gear is popular because it's cheap, but it's also cheap. I've seen several of their holsters destroyed in training; either the holster separates from the paddle, or the holster itself separates at the rivets – either way, the badguy gets your gun. I've seen quite a few IMPD detectives and off-duty officers carrying openly in Fobus rigs; it's a little unsettling.
I tend to follow Shooter521's line of thinking here. I have 2 single mag carriers. I also only feed the gun from the front one, and use the rear to reload the front during training.
Comp-Tac, Blade-Tech, Raven Concealment, and Rocketman all make fine products in this arena. Mag carriers aren't like Holsters in my opinion. You can sacrifice quality some and still end up with an alright product.
If it's something you intend to carry every day, I'd invest in one of the above companies though. The Fobus mag carriers are functional, but not as durable as some of the others. They're also not a lot cheaper.
If i carried an extra mag in a carrier i would prob go with a single because of concealment and easier to grip cleanly. (i either only have one mag or i have one in my pocket)
as for the Fobus stuff i have had one of there paddle holsters and had no problems with it at all. it worked very well in fact. so i mean go with what you are comfortable with as far as brand.
I'm very fond of the Safariland CD spring-clip mag holders. I can carry 2 @ 1911 mags and a Surefire Outdoorsman flashlight in less space than a Fobus dual mag pouch takes up. Very affordable and durable. Made for single & double-stack mags.
It feels strange to carry any other mag carrier off-duty.
Mark, I tried to insert a cellphone in the magwell of my G-22 one time.... Stopped carrying the cell on the same side as my mags.