It depends a tad on how you intend to use it...
If you intend for it to be a quick-addition carbine mag for your every day wear (in other words, going over your normal belt only when you decide to pack a rifle), then you probably want something that does not require taking the belt off to put it on. I find the Blade Tech is great for this. Also, the Blade Tech is ambi... It doesn't care which way you put the mag in. So if your left handed shooting buddy wants to barrow it, it'll work for him.
For duty use (thrown on at the begging of the day, and not removed until you undress) or such, a carbine mag holder that has a simple loop arrangement without a quick-attach means can be both thinner and more comfortable. The Raven Concealment version is excellent for this. The Raven Concealment version must be ordered in the desired configuration, though, as it will only accept the magazine in one orientation.
Now, if you're looking for a mag holder to go on a MOLLE/PALs battle-belt or similar such thing, you're looking for a different creature altogether.