My 2 cents.
Diesels are expensive to own and fix. They are not without their fair share of issues and some are much better than others.
If you plan to tow a lot and work the truck, buy a diesel. If you are looking for something to drive on the weekends and back and forth to the store, buy a gas. If you tow less than 2K miles a year, buy a gas engine.
I say this, having owned several diesels over the years, as well as gas engine rigs. Diesels are great when you put them to their intended use, which is either for economy, ie the 6.2 or 6.5 GM diesels, or for towing. They are not good at commuting to the store and putting around town. You absolutely must take them out and work them on a regular basis, or they will start to have all kinds of stupid little issues pop up, because they gum up and don't burn out all the carbon. The best saying is that a hot sweaty diesel, is a happy diesel. Work them hard, and work them often, and they will love you for a long time.
The newer auto transmissions are fairly reliable. If you are the zombie prepper type, you aren't going to find any of the newer rigs that will ever be as foolproof as a 60's or 70's inline 6 engine with a 4 speed trans, but those engines don't do well at all the other things we might like one to do, like running 80 down the freeway with the A/C blowing. If you don't plan to hotrod your truck, the chances are pretty good that a simple shift kit and line pressure increase will make it last as long as the engine.
As for engine reliability; here is my experience.
Dodge engines. The Hemi, some like them, some hate them. They don't have a lot to offer below 3500 rpm and you get real tired of listening to them at 4500 rpm to climb every hill. The MPG is fair, but not as good as the Chevy 5.3. The older magnum engines 5.2 and 5.9 are thirsty, and suffer from cracking heads, but they are fairly bullet proof otherwise. The 4.7 is a decent engine, but they have had some issues over the years. MPG of the 4.7 isn't that much better than a Hemi, maybe 2-3 mpg at best. The Cummins has it's share of lovers and haters. I can tell you that my truck, a 2002, can tow like no gas engine I've ever seen, but the MPG has never been that great. Around 16-17 empty at 70 mph and 8-10 while towing. While mine is on the lower end of what most will admit to, I firmly believe that the myth of the 25 mpg cummins is mostly just that, a myth. The vast majority of 4x4 trucks with a Cummins get between 16 and 19 mpg when driven normally, and can see 21-22 if you have 3.55 gears and a stick shift, and keep speeds below 55 mph on flat ground.
Chevy engines. There really isn't much bad to say about the 5.3 and the 6.0 engines. They run forever if reasonably well taken care of, and have relatively few issues. The older 5.7 engines are pretty good, given what they were, as were the old 454 and the newer 502, except they drink gas like there is no end to it. The duramax is an impressive engine when left stock. There are tons of them running around with 300K plus miles and they are still just fine. The Allison trans is very good when stock power is fed through it. The 4l80E is very good, the 4l60E is good, after you fix the sun shell issues and some other problems. The newer 6 speed auto seems to be very reliable with what I have heard.
Ford engines. I can't speak for all the newer ones, but the early 5.4 and 4.6 engines were adequate at best, surely nothing exceptional to talk about, other than they have good life expectancy if taken care of. It's not unheard of for these motors to roll 300K miles, but don't expect the fuel efficiency or the power that that Chevy LS based engines provide. The Diesel's? The 7.3 is a good engine. After that' it's a total crap shoot. Some have good luck, others throw the engines away and start over with a cummins or a whole new engine before they get rid of their issues. The ford transmissions after the e40d are very good, especially the 5r100.
As for axles etc. The GM IFS is a fairly trouble free system, when left alone. Lift them, and beat on them, and they will show you their worst side. The Ford and Dodge IFS stuff will never be as tough as a solid axle, but they seem to hold up pretty well to everything if they are not abused. The Ford and Dodge 3/4 tons and 1 tons have solid axles. They have their share of issues, with Ford being the better system for the most part. The biggest problem with the Dodge stuff, is that the newer ones had tie rod end issues, which have been figured out, incorrect installation led to snapping of the tie rod ends. Otherwise the Dodge's have more moving parts, and they are a little under designed for the Cummins, so maintenance has to be kept up.
The rear axles of all the Big 3 can be divided by type. Semi-Float 1/2 ton or light 3/4 ton and Full Float, 3/4 and 1 ton. The semi float 1/2 ton stuff I would rate Ford #1, Dodge #2 and Chevy #3 in terms of strength. If left stock, with stock tires, all of them will hold up fine. The chevy will be the first one to break something if you abuse it.
Full Float rear axles. They are all about even when you get right down to it. The newer AAM axles behind the Cummins and the Duramax are about as tough as any axle you will ever find under a production vehicle. The Ford is only a hair behind them.
, take it for a drive. If you like it, then decide if you can live with fixing its common issues. Every brand / model has a weak link. You've heard about all of them by now.
