Longer autofire range, much faster barrage cooldown, better RoF, better accuracy at all ranges.
The LeIG is substantially better, but only because the Pack Howitzer is completely useless.
The only reason I ever take them these days is if I recrew a stolen one and leave the squad at minimum size, for the pack or the LeIG.
IIRC the LeIG was 20/40MP cheaper, but it's been so long since I built one I could well just be wrong about that. They're both trash, is the point, and both substantially worse than the universally cheaper 120mm mortar
The LeIG is worse than the Pack Howitzer in almost ever stat, and the barrage is utter garbage.
Really the Pack Howitzer isn't useless, it just needs to be used at far to close a range to be effective. It has way more infantry wiping ability than the ISG, but that isn't saying much.