Well, I thought the original question was which was the fastest to reload, from last shot empty to first shot after the change.
For the SECOND reload, the mags have all the advantage because you're putting far more uninterrupted rounds downrange between the possibly slower reloads.
Anyone here shoot Rattle Battle much? I remember that the fastest Garand shooters could get off 16-20 aimed shots (1-2 reloads) max in the 50-second time limit, while the M14 shooters maxed out at 24 shots--1 reload.
That was years ago and nobody on the firing line then was shooting an AR type.
I guess I'm old now. Nuts.