These are obviously not purely scientific, or apples to apple comparisons, but here's what 25 seconds on youtube found.
That's one of the best helmets on the market getting dented by a shot. Notice the player keeps playing like he's fine.
This one really highlights, IMO, how much less force a lacrosse ball delivers. It also shows how easily these helmets can be knocked off even with a chin strap on. Dude takes a full lacrosse shot right to the unprotected face and he's just chilling in the penalty box after. If that's a softball he's in the ER, at least the Classic Ms we use around here.
Just for kicks and giggles...
You can see how the force of the impact is dissipated through his face and chin because of the chin strap. This is even on a high shot, not one to the jaw area. Without the chin strap on, I think the helmet pops off the pitcher's head hitting his face on the way. For anyone that still doesn't understand my point about lax helmets, this is it. Without that chin strap to stop the backward motion that helmet is straight smashing his face and flying off as it continues on backward.