It's most effective (as is the ISG) when shooting at damaged squads, but due to the Pack's larger AoE it's more likely to kill stuff at higher healths since both guns do exactly 80 damage (the health of every infantry unit in the game).
Actually, you're completely wrong(surprise surprise here).
ISG is most effective at shooting ANY squad size or spread, because it suppresses them. It also have longer range in general, meaning when both shoot target at 60 range, ISG is even more accurate.
Pack howis larger AoE means not much when it rarely hits anything because of much higher scatter values, it might wipe now and then, but its rarity(IS-2 vs Tiger AI anyone?).
Neither of them really wipes anything, but it takes a single model hit from both to win an engagement due to suppression and ISG is much more likely to score a hit, therefore its superior support unit.