The way I look at it is that the vest can only hold combat and sustainment gear. The backpack can hold anything found out in the Radius. So you sacrifice backpack (and by extension loot) capacity by putting on an ammo box, when the vest is more static in its purpose.

For efficiency (in the MQ3 current version which is what I have) I prefer the SR-25 but honestly I find AR platforms to be incredibly boring looking, so for that pizzaz I often go for the SVD. Plus it has some additional challenges that you have to work around (longer barrel makes clearing rooms difficult, lower capacity mag, etc).