Which is better, 5.9 or 6.7 Cummins?
 

I eventually decided on a 05-10 Dodge Cummins. Thing is, I wanted the 5.9 common rail. But my best friend Scotty, who believes size is everything, called me an idiot and go with the 6.7. He argues that the bigger size creates more power. I agreed, but aknowledged that a 5.9 is much more fuel efficient, more reliable, and can make the same power with minimal modifications. He argued back that you can simply remove the emissions equipment and gain back the mileage. I then rebuttled that you will void your warranty and if something goes south, you're screwed. I've got 2 trucks lined up, 6 speed manual, 4 wheel drive, nearly identical with the exception of the engines. The 6.7 is a bit more, but not enough to matter. Any advice which one is better? I'm really hoping I'm right, but if the 6.7's better, I'll admit I was wrong and buy it instead

I would just invest in a tuner if you already have the 5.9. 6.7 is larger and better but if your engine is well maintained and have a few mods it will par up to 6.7. Either of the 2 is a beast, don't worry about it. There are lots of mods you can do.