Is there grass and dirt where ever you are going to be towing?
Show grounds? Trailheads?
Where you park your trailer?
Damp grass is very slippery and loss of traction has many a 2WD truck needing to be towed out forget adding a trailer and cargo...
I would do 4WD and nothing else.
So the trucks...
1990 or 2002....
Different questions I will ask and many don't think about...
Original owners selling?{you are doing a background on it?}
Where was it registered and for how long?
How many owners?
Accidents?
Did it ever have a "TOTALED" or "SALVAGE" declared to it and was a "clean" title issued for it in another state?
Some states allow this, others do not and the laws are ever-changing so know the laws in your state...{this is where your insurance company reps helps you. Some states will not permit reregistering a vehicle if salvage ever existed to its vin.}}
Depending upon where it was registered, could it possibly been caught in flooding waters...water, especially salt ruins many components like wiring and rots all kinds of metals. Floods have been occurring all through the past since either vehicle was manufactured...
Was the state where it was registered a state where snow and ice occur? Has the truck chassis been undercoated and when, and if newer, who did it?
Are you sure under that pretty undercoating you not have swiss cheese for a frame? Is the frame integrity safe for towing a loaded horse trailer such as you plan? There is "road-safe" and there is "towing-safe" and they are not the same...
Without a solid frame under you it not make any difference of anything else...no frame no go!
Mileage on the 1990 is far less per year than the diesel when you figure it out...
Diesels "last longer" its said, they however are more to service, require special additives to fuel and filters so the fuel not gum or gel...and fuel is more costly and probably will be forever more now.
Gas engines...they get you where you want to go same as a diesel, maybe not doing 80 nor should you be ever with livestock aboard...
No one should be jack-rabbit starting and stopping pulling any type of trailer if you're smart about safety so to me toss the diesel for power in the dumpster and think about longevity....
Longevity on both of them is about even truthfully.
The frame and body on the diesel is going to require more to maintain and fall apart sooner rather than later.
The age and engine of the gas truck also has the pitfalls of age against it and needing more costly repairs due to age, period.
Depending upon how either truck was maintained and had preventative maintenance done is what my choice would be...
I can tell you though, not having 4WD is a mistake you will regret after the first "I'm stuck..." trying to get out and home no matter where you are.
Towing by a tow company...expect for them to just come to you with a appropriate sized towing vehicle is going to cost you in excess of $100 easily if you are very lucky.
Finding someone nice enough to give a pull and get you moving, don't count on it...it does happen but don't count on it.
Either truck is old...20 years or 30 years...sorry they are old and there are many safety updates that have taken place since either was manufactured.
Both are going to need more than routine maintenance because of their ages...that costs money.
Do your homework and check those car reports, car-fax reports and speak to your insurance company as they can run some stuff you will not be able to index.
Based on age alone, the 2002 even though a heavier truck so more expensive to plate and insure may also be about the same to insure as the lighter one because it has more current safety features mandated on it...
Much to think about...
For me/us...no 4WD is a deal breaker and not even considered when truck shopping.
For reference..
We had {long ago sold} a 1993 F-250 HD 4WD long bed, stick shift truck with a 351 gas engine...was a beast towing.
Towed a 8,500+ pound trailer up and down mountains, through deep sand and of course on paved roads...
It averaged 10 -12 miles towing and about 15 -17 around town for reference...hubby often threw a chain to the back bumper of the trailer and pulled buried/stuck 2WD out with no problem...
Go with getting 4WD, even if not this vehicle if you decide to pass on them for any reason...my strong suggestion.
Happy shopping and enjoy the journey.
🐴...