Stormtroopers hit 3 shots in 8. So 5 of them will get 1.875 hits.
Rebels block 2 in 6. So 1.25 of them die.
Rebels hit 4 shots in 8. So 5 of them will get 2.5 hits.
Stormies block 3 in 6. So 1.25 die.
It's exactly the same. The difference is in keywords: Stormies do better aiming, and Rebels do better dodging *if* more than one thing is shooting at them this turn (but that's almost always the case for important squads, so...).
Long story short: heavy weapons aside, a *vanilla* squad of rebels and a squad of stormies can shoot at each other for quite a while before they'll wipe each other out.
Heavy weapons, though, should roughly double the kills if they fire, and the Stormies have the advantage there of having the really good heavy rifle for flexible anti-infantry duty. On the other hand, they need to have decent anti-armor somewhere else in the list if they don't bring the missile launchers, so swings and roundabouts.
Remember that if you have a point of suppression you get cover - that's important. It's one of the things that makes Rebels pretty resilient with dodge, since the second attacker is getting two hits removed, even in the open.