I believe they are two different Rolling Block action sizes?
Obviously one larger than the other.
The Uberti is the smaller of the two, hence the lighter weight.
I'm not sure about the IAR model.
but in looking at the IAR web site, it sure looks like the smaller action
They're all Italian made rolling blocks, and it's no big
surprise to see different distributor's names on a firearm made by
Uberti. Give them a phone call and ask 'em . . .
Can always give each outfit a call and see what's up with 'their' models,
for more specific information and details.
The lighter weight one would be my choice. Why a larger action than is needed, for a .357 round? Just extra weight to haul around in the woods.
Hope this helps some...