Though I have a lot of respect for the F150 Ecoboost truck, this one is a no brainer, in every category of consideration you mention -- buy the diesel!
I bought a 10 year old clean, one owner Ram CTD two years and a few months ago, it tows stably and effortlessly, and though I paid a bit of a premium for it compared to what a comparable gas half ton might have cost, my truck hasn't really depreciated at all, which is quite amazing! Further, as others have said, crawl under the Ram and look around -- all components are impressively heavy duty and rugged. This is my first diesel and I am sold on them -- I wouldn't buy a gas truck for any towing duty.
