Which will save more..

[Tropical Orb]

Hey, I have a quick question:

My trip ends in Lisbon, Portugal and begins in Rome, Italy. I'll be flying into Roma from Los Angeles. To save money, would it better if I bought a one-way ticket to get to Rome, and a one-way ticket from Lisbon to Los Angeles to get back home. Or should I do roundtrip - Rome to L.A and back, flying into to Rome from Lisbon to make my flight?