Yep, the 6.4 is a nice setup if it meets your needs for the trailer. Of course, there is the endless gas vs. diesel debate but most folks can somewhat agree around 10,000# is the time to start considering the diesel. That being said, I'm hauling a 11,000# 5th wheel with the less capable rear end ratio (3.73) and it generally does fine. Also worth noting I live out west and regularly have to tackle hills.
Also, X2 on the Dennis Dillon recommendation. Even if just for comparison with your dealer and to use as leverage. When you look at their site don't fall for the "as low as price". That requires you to belong to a bunch of associations that you don't. The "sale price" is still very good.