Anything with the carbine system will have a shorter elapsed time until the gas hits the bolt carrier and begins the cycling process than rifle gas system will.  That means that pressures will be higher when the extraction process begins, which can lead to extraction problems.  That is the primary difference.

The second, but less important issue is the amount of barrel beyond the gas port.  There has to be enough barrel to sustain the gas impulse, or not enough gas will be directed to reliably cycle the rifle.  This is really only an issue for 10" or 11.5" "carbine system" or 16" "rifle system" guns, because they have such little barrel beyond the gas port.  This is why Bushmaster and others add a second gas block at the carbine location for their Dissipator rifles.

rtl,

To answer your question: 20" barrels with the "rifle" gas system will be the most reliable.  However, the carbine system has been refined enough that there isn't enough difference that you should choose your rifle's configuration based on them.  Instead, you may do better to focus on range issues (i.e., how much barrel you need), weight, and sight configuration, all of which are much more important in the scheme of things.

-Troy