In case of IS you have to take into consideration that all their weapons are useless out of cover so they need maps that give it and micro while grens can be a-moved to victory on any map. So in my opinion grens win big time just for being consistent and the ability to fight flamers (in cover its just a death sentence) itself.
Cover givEs section extra Vision at vet1. Non vet bonus is +50 rof. You think that, it really hurt brens? Lets make 3 vet lmg grens against 3 sections with double brens. Even both without cover. Guess who will won? Also, grens are not allowed to build any piece of green cover so what we are talking about