Both governments have the power to veto. Veto means "i forbid" in the United States only the president has the power to veto. In a roman republic only the two consoles have power to veto.
Both governments have a checks and balances system. That means no branch in the government can take all of the power
Difference #1
The legislative branches are different because in the United States we have a house of representatives and a senate ( congress). In a roman republic there is a sentence and a assembly
Difference