Welcome to the ORG.
First question, do you intend to delete emissions control equipment?
Reason I ask, if you do not intend delete the DPF system on the 6.4L, you will want to go with a 6.0L. That DPF is the critical flaw of the 6.4L, leading to premature wear on the engine and a major loss of fuel economy.
The shortfalls of the 6.0L are well known, well-documented, and the aftermarket has answered up. When you consider the cost of the two trucks, with the simplest reliability upgrades and tunes, you will have less invested in the 6.0L.
The only thing the 6.0L requires is a monitor, and an owner that does his research before and during ownership. Where as a 6.4L requires the monitor, and a DPF delete and tuner (can't delete with stock programming) to have respectable fuel economy.
Both transmissions are the same design, interiors are a matter of preference.
If you decide on the 6.0L, start doing a lot of reading here on the ORG, and know what you are getting into. Its not the horror that some claim, but it requires a few things. Send me a PM if you'd like more information.