TL;DR trukz are cool
I'm somewhat biased because I'm on my second one, but, 6.7 Alumiduty for me personally. I plan to upgrade to one from my current 6.7 early next year.
First of all, why do you need an F-450? I probably do the most "heavy hauling" and regular trailering out of most people I know, and I get by without issue with a SRW F-350 with air bags in the rear. If you want a dually, by all means go for it but 450’s ride like haywagons and get shit mileage. If you’re not towing excessive shit regularly I wouldn’t bother.
The trailer I'm towing weighs 8,700# empty, truck is easily another 7,000#. Front is actually about an inch higher than the rear in this picture with 30#'s in the bags.
Secondly, they all have their plusses and minuses
RAM- Cummins engines just work. They're reliable, use tried and true tech and really don't have many common issues. However, when you try and make decent power on the 6.7's you will run into head gasket issues. Simple solution, ARP studs and you're done. They still run CP3 injection pumps which don't have the horror stories associated with CP4's like the Dmax and 6.7 Powerstroke run. Overall I think the Ram is a good chassis, solid axle front, ride like a "truck", no real complaints there. 68RFE transmission is a different story. They are getting BETTER, but not refined yet by any means. Tuners are still trying to figure out the best parameters for them and rebuilders are finally getting a grasp on what makes them live. Expect a built 68 to cost you somewhere in the neighborhood of $6,500 if you're planning on making any real power.
Chevy/GMC- I don't have a ton of experience with Duramax engines, but have a ton with GM HD trucks. What I do know of the L5P is they are good engines overall, I was hearing some horror stories early on but haven't lately. Biggest downfalls to me with them is they are just so new you're kind of along for the ride figuring out all the little shortfalls of them in years to come. Deletes and tuning are ungodly expensive, still running $4,000+ for the whole package. Runs an updated CP4 style injection pump, while they're better than they were at first, there still is that risk of a complete system contamination from a pump that comes apart. $8,000 ish job depending on where you go. Allison transmissions are good, do what they need to do and don't have many headaches if you're not a complete asshole driving everywhere you go. Never had any complaints with the chassis aside from IFS if you're using it for plowing, even still not bad, just don't take the weight of heavy ass plows all that well compared to the others. They ride the best out of all of them hands down.
Ford- I personally love the 6.7 PSD's. I've got about 150,000 miles of pretty strenuous use between the two that I've had and have never even seen a CEL come up on my trucks. They've been around relatively unchaged since 2011 and have proven themselves to be very reliable. Yes, there are horror stories like anything else, but compared to the last two engines Ford pushed out, they're angelic lol. They also suffer from the CP4 pump failures, but less than the duramax trucks IMO, which I believe comes down to the fact that Ford installs an in-line fuel pump while Duramax trucks rely solely on the CP4 to suck fuel through the system and then pressurize it to rail pressure. They make great power stock or tuned. The 6R transmissions are great, I personally favor them over the Allisons. My truck gets the absolute shit beat out of it daily driving pretty much maxxing out stock air and fuel systems, and tows 15000#'s plus regularly. Currently sitting at 170,000 miles stock trans and factory sealed engine. I also think that Ford makes the best chassis, that's simply opinion but having owned a newer RAM and newer fords, they just seem more solid to me.
Hope this helps lol.