I live in Orange (near the Tustin city limits) and work in Aliso Viejo.
The others are right, the difference in riding between Irvine and Newport is small, so make your decision based on other factors.
One thing to note is that it is hotter in Irvine during the summer, how much hotter depends on how far inland you live.
Personally I wouldn't live south of Irvine, not because the communities there are bad or anything, but because being further south would make me more far removed from my family and other activities.