I've had the 40 and currently have the VORTEX. The VORTEX is deeper and drones less than the 40. The 40 is louder but annoying as balls on the interstate.
Here are some VORTEX clips.
And a review I did. Feel free to ask questions.
Don't get me wrong, the 40 sounds nice, but the Vortex sounds better IMO. If you are gtoing with flowmaster, I'd look into the 50. It's deeper than the 40.