keep in mind resale value too. A 24' will sell for a lot more than a 20' will if/when the time comes to upgrade or if for some reason times got tough and you had to sell it.
One nice thing about a 20' is that it's right at the edge of what most 1/2 ton trucks will pull. A 24' almost requires a 3/4 ton to pull it safely. I've pulled my 24' with a 2003 Lightning, and it had plenty of power but where it suffered was (1) brakes and (2) stability. When an 18 wheeler passes you on the freeway, or if the wind is blowing any, a 1/2 ton just isn't safe. Lot of guys do it, and again I have too but going to a 3/4 ton was a BIG improvement!
Also look not just at towing capacity, but also at how that tonnage is rated. For instance my truck has a 12,500 lb towing capacity. But that's with a weight distributing hitch. Pulling on the ball only, it drops to 6,000 lbs. My 24' loaded with all the junk comes in right near 7,000 lbs, which is the trailer's max weight.
And speaking of weight capacity, take a good hard look at the 20' trailer's max weight. A typical fox body is going to be around 3000 lbs. Plus tools. Plus canopy. Fuel. Generator. All the rest of the stuff you might carry. My trailer's max GVW is 7,000 lbs with a 3500 lb empty trailer weight. A door car, tools, generator, fuel, winch, battery, spare stuff will put it really close to maximum. I kind of wish I'd have gotten a trailer with 5200 lb axles but this works for no more than I race anymore (twice a year maybe).
Just my $.02, for what it's worth.
'92GT, 427" and '93 "LXVO" coupe