Think of it like this...
The 308AR has a longer carrier than an AR15..So it needs more room to operate. DPMS and armalite went about solving the issue 2 different ways.
DPMS decided to use a standard AR15 buffer tube with a special shortened buffer.
ARMAILITE decided to use a standard AR15 buffer with a longer buffer tube.
For rifle length stocks, they both use standard AR15 rifle extensions with shortened buffers.
You can use either complete system on either style rifle, just be sure you aren't mixing and matching.
Not sure there is really a best weight. All these guns are different and a little quirky. Most like heavier buffers, some don't. You can run a carbine setup on an 18" mid no problem.