I always ride out and back. I'm the type that can ride the same route everyday and feel fine about it. But generally I have a 26 mile course that is flat that I'll ride M-F. Then as I get stronger I ride a 30 mile hillier route. But my favorite is the same hiller 30 mile (round trip) out and back extended to 50 miles (round trip) out and back. I start in Cen-Tex and end up in West,Tx before I turn around. Not the region of Texas but the actual town called West.
As I generally consider all my rides training rides, out and back gives me more feedback ie wind,hills, negate themselves and I get a more honest ride and time.