Vet differences make both of them equally or more durable actually.
Bear in mind that it doesn't matter how many rifles there are, but the final squad DPS. For example grens and volks have almost the same DPS despite having different number of men in squad, therefore volks are considerably superior for only 10 more mp, however they fall short with veterance and upgrades taken into consideration.
Does the Stg44 upgrade not shift things into the Volks favour? I thought it might have, may be mistaken however