I am in the same quandary and have been investigating very heavily for months. I really would like to purchase a Toyota due to their inherent reliability, but the Tundra is about a generation behind GM and Ford in the heavy duty 1/2 ton truck department in many areas including towing, fuel mileage and options.
My 10.4k pound equipment trailer would only be towed rarely with its max weight and seldom with a lesser load. But I want a truck that is rated to tow my maximum load with as much head room as possible. So the truck would mainly be used without towing and not as my primary vehicle.
I have driven a Ford 3.5 Ecoboost 4x4 with max tow option, along with a Chevy 5.3 4x4 with max tow option. The Chevy rides and drives more like a car while the Ford drives more like a truck. The Ford EB has more acceleration at any speed, but the Chevy is close. Neither OEM is willing to provide > than ~ 10% off sticker for a 2015; 2014s are a lot less but do not meet my feature and fuel mileage needs (Ford). Sticker pricing is $52k (Ford Lariat+) and $49k (Chevy LTZ+ and GMC SLT+). I currently have AAA buying service getting me pricing on all three.
But since my heavy duty usage is seldom, I may just go back to my original plan of a quality used, Ford or GM 3/4 ton, 4x4, relatively low mileage, maybe 2008 - 2010 with a large V8 gas engine. I also have a large investigation packet on these.
If tow rating is important to you, consult this as a quick reference guide
Trailer Towing Guides | Trailer Life Magazine, which was instrumental in helping me narrow down my choices to where I am at now.
I would prefer the Ford for a bit more towing and carrying capacity, a bit more power and rated gas mileage and the ability to also have the off road suspension in conjunction with the max towing package. With Chevy or GMC one can have either the max towing or Z71- but not both combined.
Edit: I prefer the GM products as a daily driver.