I have never owned a Ford but plenty of friends do and they don't complain about having problems. I have ridden in them and they are comfortable,
My previous truck had the Denali package and was a nice truck. It didn't have any options that couldn't be purchased on a Silverado LTZ or Sierra SLT aside from a few appearance items. This is also the reason I ended up with the Silverado this time as it was a few thousand cheaper than a similarly equipped Denali.
I don't think you can go wrong with either truck. If money isn't a concern pick out the 2015 you like best and enjoy.