The Dodge is the better vehicle from the standpoint of suspension/chassis. That torsion-bar front and leaf-sprung rear is better, any day, than a coil all around GM for any kind of handling. Towing only maginifies the superiority. KONI shocks are called for, as are new rubber bushings.
And, while the V8-318 is an excellent engine, the pre-1967 version is not the same beast (this bears checking out). The motor would need to re-timed (engine mechanical advance) and a vacuum secondary/spread-bore carb would be appropriate. Unleaded gasoline will take a toll on the valve seats (high loads, long times, higher than normal op temps and elevated cruise rpm). Frankly, I'd go with a more modern 360. They've been bullet-proof in motorhome combinations. Same with the transmission. The electrical system needs upgrading to a more modern alternator and starter, and the ammeter needs to be bypassed.
The one true failing is brakes. Single circuit is, IMO, unacceptable in a tow vehicle. Drum brakes are okay (just, they bear a lot of PITA time in getting them adjusted properly). There are plenty of aftermarket conversion kits (STAINLESS STEEL BRAKES, for one) that can address this properly.
Having had some old cars I believe that re-wiring them is the best solution to the worst headache: whether in the 1950's or the 2000's, the reason 80% of cars are in the shop at any given moment is electrical. Rewiring all but the dashboard isn't too terrible a job.
Last, seatbelts. It needs three-point harnesses.
You can ask plenty of questions at the: C-Body Dry Dock and get more (likely better) answers. (Contact Mark Olson for a 1967
Chrysler Tow Guide that diagrams the necessary way to fabricate and install a hitch). Same at mopar.org
The Cadillac is the easier choice. The drivetrain is almost as good as the Dodge for longevity. It still needs emission equipment removed (not all, as some is beneficial to operation and longevity) and it needs the engine timing and fuel delivery modernized for efficency. That vehicle ought to be able to get 16 mpg, solo, on the Interstate. Keep at it until it does.
The brakes, the chassis are suitable for towing. The rubber bushings all need to be replaced, and KONI shocks installed. The brakes need a new master cylinder, vacuum booster and hoses. Re-do all the calipers and rear drum hydraulics.
Near thirty year old wiring is the reason I'd be a bit leery of this car, as there is so much of it. And, unlike the Dodge, working on it is a long and big job.
The real advantage of the Cadillac is the steering, as variable-ratio is superior to the Chrysler. It also has a longer wheelbase. I'd add a rear anti-roll bar if not so equipped.
I think you can have fun with either one. The Cadillac is the easiest choice given identical states of running reliability. I'd rather have the Dodge.