What does your pocket book tell you? Your Dad is right, or sorta right, or maybe not. A diesel can potentially be a heck of a lot more money to have as a daily driver, or just marginally more expensive, or barely more expensive. I wouldn't own one of the newer ones out of warranty personally. Diesel fuel costs more than regular here, problems with them are typically way more expensive to fix.... etc.
Any of the gas HD 250/2500s will handle that kind of weight without issue. I wouldn't hesitate to run up to about 10K with mine and not think twice about it. It's pulled that weight plenty already so I know it manages it just fine. Realistically, almost any modern 150/1500 would handle that without issue too as long as you don't get the super bling models.
You don't need a diesel- heck, you don't even NEED a 2500 but that's personal preference. I have a 2500 for a similar sized trailer and a large family so I won't pass judgement :)
I think you'd be kicking yourself in short time if you went diesel for that. They have their place, but this isn't it. Unless you just want one for the sake of having it- then I say giver!
