The original Flowmaster 40-series is the deepest and loudest muffler available for street vehicles from Flowmaster. The Delta-Flow variation was redesigned for improved power gain over it's non-Delta-Flow same series cousin. If you compare the Original to the Delta-Flow, you can see how the Delta-Flow would have better flow characteristics, yet the non-Delta-Flow makes for a deeper sound.
The original 40-series is significantly deeper sounding, but in the higher revs gets a little too raspy for my taste. jhrody has it on his 3.0 stratus coupe. The direct-flow gains a little bit more power, but as with most small mods, you're unlikely to feel a difference.