My understanding is that the SCV6 is the exact same motor as the V8. They just left two cylinders blank. So the timing chain is the same. I haven't noticed my 2016 SCV6 idling rough, but I have never been in the 8 cylinder model, so I can't vouch one way or the other. I find the engine to be very truck-like, a bit coarse. But it seems to have plenty of power for my needs, and the transmission is fantastic. I do a lot of highway driving and often tow a 3,000# boat.

 

Correct...same block less 2 cylinders. Power output and where it comes on is different in the SCV6. I drove both & thought the V8 engaged more quickly. 90-ish% of my driving however, I probably wouldn't notice.

 

The ZF 8 box is reason enough alone to go for the later V6 in my view. It vastly superior to the 6 speed. I've had the 4.4 V8 (amazing engine but not quick). I now have a 2016 Landmark and they're like night and day in terms of pick-up and the gearbox is outstanding. I'd go for the newest you could afford and if that gets you the 8 speed I'd personally recommend it hands down.