From what I've heard and seen on some flow charts a crush bent pipe isn't the most efficient way to go and a mandrel pipe should flow better keeping the exhaust flowing without creating a large amount of turbulence. I'm not sure if it would matter as much in a dp but even in the y pipe and single pipes a crush bend will cause some unwanted turbulence.
If I can find any of the flow charts I'll post them here and let you make your own decision.