I have owned both, a 2009 6.4 and a 2012 6.7. Never had a problem with either but will say this, it is not the engine BUT the transmission (5R110 in 6.4 and 6R140 in 6.7) that makes a big difference. The 6R140 is much more capable and helps with the MPG along with the new emaission system that uses DEF. I actually preferred the twin turbo setup better in the 6.4 and there were other things that I preferred about the 6.4. The 5% bio diesel is more of a limitation of the emission system rather than the engine.