Let me start off by saying I am a dodge owner but I own a diesel performance business and I will give you the most unbiased answer I can.
If you buy a ford, buy a 7.3. I just saw a 06 6.0 with 300 miles lose a turbo. The 7.3 PS is a tried and true international motor, the same block as the DT-444 motor used in the international trucks for decades. I own an 05 cummins and you could not give me a ford but I will tell you the 7.3 is a good motor. If you want a ford with a cummins, go to fordcummins.com. Cummins is a basically a small road tractor engine that will run forever. As far as a duramax goes, I would not own a truck with a diesel engine that doesn't have a solid front axle.
There are a few problem areas with the dodges you need to watch out for. If you buy a 24 valve truck from 98-02, watch out for the injection pump goin out. If it has over 100k miles, make sure it's been replaced. The dodges are also notorious for having a "death wobble" in the front end. They are bad to wear ball joints. So take your pick, either worry about simple things like the injection pump and ball joints, which in 3rd gen trucks (03-06), are much more reliable (The pump was changed from a vp44 to a cp3) or you can worry about things like injector failure due to a new design that works off of high oil pressure, turbo failure, head gaskets, heads cracking, computer failure, etc. The 6.0 has almost put ford under. The newer ones have improved, but they're still not reliable at all.
As far as auto or manual. Buy a manual if you plan on towing heavy. Being able to control what gear you're in helps a lot. You can also use an exhaust brake much more efficiently, and yes they are worth every penny. On the other hand if you buy an auto truck, you can buy transmissions that are built to handle it. I am a dealer and I sell suncoast autos that have an unlimited mileage and horsepower warranty. If you are like all the rest of us, you will catch the horsepower bug and want to see some black smoke rolling, so you will either run into clutch slippage or tranny slippage if you start going over 100 hp over stock. If your clutch starts to go, you're lookin at $1000 for a good clutch, but if your auto tranny starts to go, you're lookin at over $4000.
If you want to know anything else feel free to send me a message. I will answer any question you have, and if I can't answer it, I will get an answer for you.