There are a few indicators of a mild vs. severe curb bit. Any and all of this essentially refers to the potential
severity -- any bit, even if it's made of braided silk, can hurt a horse's mouth with the wrong hands on the reins. A vaquero-style spade bit, while scary looking, can be a phenomenal and gentle communication aid with the right hands on the reins. Harsh bits don't ruin mouths, harsh hands do. Shank vs. purchase ratio:
Long shanks with a short purchase (length of the upper part of the shank, between the mouthpiece and cheekpiece of the headstall) typically indicates a harsher bit. A more equal ratio indicates a milder bit.
Curved or swept shanks are generally milder than straight, at least in that there's a slight "delay" in the activation of the leverage action that isn't there in a straight shank. Straight shank = quick response, but it's easy for bad hands to overdo it and intimidate/frustrate the horse. Mouthpieces:
Thinner is generally harsher, but don't jump on the thickest one out there in the name of mildness. Consider your horse's mouth conformation -- if he's got a low palate or is otherwise lacking space in there, a thinner mouth will probably be more comfortable.
Jointed mouthpieces tend to get confusing when mixed with leverage action -- I'd avoid a jointed mouth unless it's of the Billy Allen variety (a limited-range swivel joint -- almost no potential for pinching).
Ridged or otherwise textured mouths are more severe than smooth. Twisted wires fall into this category. If you've done your homework and prep work properly, there's generally no need for these kind of mouths.
Higher, narrower ports are generally harsher than lower, wider ports. American Tom Thumb vs. Argentine:
The main difference here is in construction and balance. Every TT I've ever laid hands on has been a poorly made, poorly balanced hunk of junk, and almost every rider I see with one is hauling on it two handed like it was a loose ring snaffle. Argentines, with the slight sweep in the shank, have a little delay to the action. A solid mouth with a low port eliminates the "noise" of the jointed mouth and the mixed-signals that leverage on a jointed bit can send by accident.
Given these very general
"rules," a good bit to look long and hard at to transition your gelding out of his snaffle (assuming that he is soft, supple, and neck-reining well in the snaffle) would be an Argentine with a low port, or perhaps a Billy Allen style joint. However, every horse is different, and your fella may make his preferences quite clear to you once you start experimenting.
Incidentally, that's interesting how your 4H rules are shaking out on the subject... I was a member for years, both English and Western projects, and I've been a club leader for a few years now. In my state, Western horses don't have to make the switch to a shanked bit until their 5 year old year. Even then, I do believe that gaming horses (barrels, etc.) are still allowed to run in snaffles (although most are outfitted in comparatively harsh curb half-breed rigs...) 3 just seems awfully young... most horses in my area are barely started under saddle at that point, and would be nowhere near prepared to "graduate" from their snaffles.
Anyway, hope that's helpful to you!