I cant speak for the diesel, but I have had both the Hemi in my 2014 Summit and now the V6 in my recently purchased High Altitude.
I did not consider the Diesel mainly because the cost of it, I hear lots of stories about issues with it and I guess other maintenance that isnt normally required on the gas models, plus gas is dirt cheap right now. Diesel costs more than premium typically.
So with that said here are my thoughts on the Hemi. First off the power and interior cabin sound is awesome! Just under normal acceleration you can tell there is something big powering it. Fuel economy is ok, certainly not great. if you do lots of stop and go its gonna be bad. If your are in far suburbs it will be pretty much as advertised about 16-17. In winter even lower if that applies to you. My Summit with the V8 felt heavy! You can tell it weighs more just in everyday driving. It has hydraulic steering which requires much more effort. Word has it the '16 Hemi will be equipped with the same electric steering that the current V6 has which might make it feel lighter.
I nowe have a new '15 High Altitude V6 and really like it so far. In everyday driving the V6 and overall interior is much more quiet. Thats not road noise or anything, but the Hemi puts out some rumbling during acceleration that is much more quiet in the V6. I know the High Altitude has a sportier suspension than the regular air suspension, but my V6 HA drives MUCH better than my V8 Summit did. That thing felt like a tank where as my new HA feels more like driving a sedan. It was that nice that I had no problems giving up the sound and power of the V8.
I only have 150 miles on my new HA, but the tranny is very smooth. no twitchy or jerky shifting whatsoever and thats even in some extreme cold temps. One thing that kind of annoyed me about the 8spd in my Hemi which was awesome BTW, the 1-2 shift is always very noticeable. In the V6 is smooth as butter so far which I really like.
The V8 recommends mid grade fuel (you can use regular if you want to), the V6 obviously regular.
Like I said before I have no experience with the diesel, but I would recommend getting the V6 again. Not sure if they did anything different with the tranny or software, but my recent built HA shifts excellent (knock on wood).
Here are a couple pictures of Billet Silver High Altitude when I was at the dealership buying mine. It was a tough call between the white and silver, but this silver one didn't have HK audio and that was a must. Also a picture of the white one that I bought with the silver one behind it to compare.