I have a diesel, 2003. Yes I changed the fan belt recently. And water pump. Under an hour for both. It's not as hard as people make it out to be. Unfortunately, once the D word (diesel) hits, the price for repairs goes up. Water pump on mine was $174 and the belt was $48. I have changed a water pump on an old 351w gas burner for $50 today's cost-including gaskets and coolant. For that matter, I also did my 2003 Lightning (5.4 supercharged) water pump for cheap-using motorcraft parts and coolant.
Anyway....
if you plan on keeping it, make sure to keep some sort of warranty on it, unless you enjoy forking out a lot of money down the road for repairs. Gas or diesel, but diesel obviously costs more.
Diesel....you have DEF to deal with; among other things. Personally after owning and running diesel pickups, I don't see the hype (and yes it's mostly hype). I think I'd just as soon go back to a gas burner; and honestly unless you're pulling a load for a living, diesel engines aren't needed. But some folks just have to have the biggest most powerful engine to satisfy their ego or whatever. I understand that, and don't get me wrong I'm not bashing anyone...that's just the way some folks are. BUT the thing about the diesel engine is resale. A gas burner is going to resale for a LOT less than a diesel will, and I mean a LOT less. Out here, my exact truck with a V10 gas is going for around $4500 with similar mileage. Diesels are going for around 10,000. That should tell you something right there.
Ford vs chevy. IF you get the chevy, make sure the warranty is good on the engine. We have an 07 at work (shop truck) with 127,000 miles on it as of today. It's had all 8 injectors replaced, high pressure pump, alternator, transmission twice (Allison which is supposed to be "bulletproof"), wiring harnesses, 2 glow plugs, fuel line failed at one point while I was driving it, 3 sets of brake pads. That's all I can think of right now. The turbo vanes are sticking so we may end up having to replace that ($4000) in the forseeable future. Batteries are 2 years old (Delco OE replacement) and they're nearing end of life (slow to crank). We've put almost $13,000 into the truck in repairs...more than it's currently worth. We will not buy another Duramax. It is the most uncomfortable, worst riding and UGLIEST 1 ton I have ever driven. When you first sit down in the seat, it feels good. After about 10 minutes, I find myself trying to move around to a different spot to get comfortable, and I can't. Not much leg room. Transmission can't figure out what gear it want's to be in. I'm 6'5 so I need leg room. The A/C only works sometimes; sometimes it's going nice and cold, and a minute later you're burning up, then it's back to cold again in maybe a minute, sometimes 30 minutes. This one does not have DEF. I like how quiet it is, and it's got power, but it's also come at a cost. Prior to this we used a 2003 F350, 7.3 auto. Had it since new, put 118,000 miles on it. Alternator was installed at 43,000 miles and a wheel / hub bearing failed at 93,000. Those were the only two issues we ever had. OH I forgot about the glow plug relay too, which I personally did mysel using a Stancor relay; thinking that we were keeping the truck. Boss sold it and bought the Durmajunk thinking that the old 7.3 was getting some miles on it. Wish we'd have kept it honestly.
Dodge, you buy an engine and get a piece of truck with it. I have not been happy with Dodge (now Fiat) trucks in many years, other than the engine...and even then, I haven't been all that impressed with them. At one point I worked for a wrecker service and we had 2 06 Dodges and 2 06 F350 wreckers. All the same trucks but different brands. The Ford's would run off and hide from the Dodge's. Wasn't even close. I didn't care for the inline 6's slight vibration at low speeds either; but that's personal preference. Some enjoy it but if it was my truck, I don't know if I could get used to it. Also on the inline 6 cylinder, they aren't real powerful unless the pipes are packed (turbo is making boost). That's one reason they went to a VGT turbo, so that you get boost right off idle and retain some top end power. Ford did it too on the 6.0's; and it works good but I felt that the 6.0 had more power. I am aware of the 6.0 and it's "problems", but honestly they're better engines than people give them credit for. They got a bad rap from the early 2003's; as well as those who run the poo out of them and never maintain them. We never had a minutes' problem in 243,000 miles, at least that's what one of the wreckers had when I quit that job. I think the other one was closer to 200k. The dodges had around the same mileage give or take a few. The owner of the company wouldn't buy the GM trucks because of the expense to repair them. At one point GM (or Isuzu) extended the warranty on the injectors to 200k miles but that was only for personal use as I understood it. If it were to be used as a commercial truck, it carried the normal commercial warranty, which applied to wreckers. May have changed since then. That was in 2009 when I quit. The injectors for our current shop truck ended up about $600 EACH, plus a ton of labor to disassemble half of the top of the engine to get to them. Could have gotten them cheaper elsewhere but also takes longer and we'd still have to pay someone to put 'em in. I'm done with GM products after this deal with the shop truck. We are going to replace it and I am pretty sure the boss has decided to get a Ford this time, and likely a gas burner. I don't blame him, not one bit.
As a personal, non-commercial owner, I would also get a gas burner. And I'd also get a Ford. Ford perfected the overhead cam rather than trying to use the cam-in-block design and then adding variable valve timing to it....those GM engines use oil, it's considered normal. One of my coworkers has one and at the 7500 mi interval, it's down about 3 quarts. GM won't do anything about it, saying it's considered "normal". My dad's got a 5.4 powered 3/4 ton and I honestly believe it's got almost as much power as my 7.3 does. I'm impressed with it, highly impressed. It's towing fuel economy isn't great (about 11 mpg) but my diesel's MPG towing is 12.5, so there's not a big enough difference to justify the extra costs. Unloaded, he's seeing 15 mpg. Mine's mid 18 range and fuel costs more; so it evens out.