I like the 50 as far as tone. You may want to go with their new Cat Back system based on the 70 series though. It is tuned for our trucks and the guys that have them have not noticed any decrease in low end torque. I had a custom exhaust installed using the 50 series and lost some low end tq. I am hoping they will come out with more Cat Backs from Flowmaster based on the different series mufflers.
If you do go with custom exhaust then I say 50 if you like it a little louder but still want to hear your radio when going down the highway at cruising speeds. If you get a 40 you will be in drone city they are really loud.
I have the Flowmaster 70 dual exhaust kit (click here for my install post). I like the deep tone of it and haven't noticed any loss in low end. I have heard a custom 40 series and I remember it being too loud for my taste (idle and under heavy throttle). The 70 is progressively louder than stock the more the RPMs go up.
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