The silverado is not an upgrade from the Ram. They're both very similar in terms of engine power, transmission gears, suspension, etc.
A 2500 just doesn't cost that much more and would be the proper truck for the application. It may even cost you less if you don't have to upgrade tires, wheels, add airbags, etc. And it may hold resale value better.
Diesel is nice but is completely unnecessary for a trailer of that size, especially if you aren't pulling often. The RAM 6.4 or the Chevy 6.0 are both more than capable of pulling that weight.