I have had the Dmax since 2008 and now on my 2015have it again. I really don't care about all the hype in engines as long as the truck drags my 40' TT all over the country with no problems, strain or pain.
I really like the Sierra truck and have the loaded version. People get all wrapped up in the engine and forget that you are in the cab for hours on end and at the end of the day the GM trucks ride great and the their interior are the best bar none. IMHO
So bottom line if you want the truck do it or if you can wait get the last of the current engines as they should be a few bucks cheaper.