HOn30 is similar to On30 in that it uses the track of a smaller gauge (N) to represent narrow gauge. Small N mechanisms are usually used as the basis for HOn30 locomotives.
HOn3 is a separate gauge unto itself (10.5mm vs 9mm for HOn30), accurately modeling 3ft gauge prototype in HO scale. While some N gauge mechanisms can be widened, and some HO mechanisms narrowed, those modeling in HOn3 tend to use mostly locomotives specifically made for HOn3. There are a few geared locomotives that were made in both HOn3 and HO, as was the prototype. The Roundhouse Shay is an example. The Keystone Shay was/is made in all 3 gauges - HO, HOn3, HOn30.
Because for many years HOn2 (would be 7mm gauge) was deemed to be impractical, HOn30 is often used to represent 2ft gauge prototypes. There were very few common carrier 30 inch gauge prototypes; 3ft gauge dominated the choice of narrow gauges, with 2ft gauge being next most commonly used. Or due to the use of N mechanisms, free-lance (realistic could have been) models are made.
On the other hand, when Bachmann came out with its line of On30, it chose to use 3ft gauge prototypes for its models, with the gauge narrowed to run on HO track. This being the case, some On3 modelers widen the gauge of the Bachmann equipment to run on true On3 track. There are those who model 2ft models in O scale (On2) using 1/2" gauge in 1/4" scale.
There is commercial flex track available in On3, On30, HOn3, HOn30. Turnout selection is more limited in the n3 gauges, but because the n30 gauges use HO and N gauge, no problem there. There are 2 brands of pre-made commercial turnouts in HOn3 - MicroEngineering and Shinohara. There are numerous brands of kits and custom turnouts as well.
The real issue, depending on the modeler, is the lack of RTR locomotives in any narrow gauge but On30. On30 has quite a variety to choose from. HOn3 has but three RTR non-brass locos at present - the Blackstone K27, the Concor Goose, and a free-lanced vehicle similar to a Goose. There is an active used brass market for HOn3. Available loco kits include the aforementioned Shays, Roundhouse 2-8-0s in inside and outside frame versions, Grandt's tiny diesel, and an unusual cab forward 4-4-0 used by the NPC. The Roundhouse kits are out of production but regularly show up on eBay.
There are loads of car kits in HOn3 in plastic, metal, resin, and wood. Very few kits are Athearn BB style shake-the-box; many require several hours to assemble.
I can't tell you much about Hon30 locos or rolling stock because I haven't sought it out. Roco does make a line of RTR.
Both HOn3 and HOn30 have plenty of structure kits and details to choose from (more than On30), as all the older prototype standard gauge HO stuff is useable. HOn3 cars are about 60% the size of On30. A model of a 28ft car is just over 4" long in HOn3, and just over 7" in On30. Microtrains N couplers are accurate scale size for HOn3, they and fine-scale wheels are sometimes used in HOn3 for greater realism.
my thoughts, your choices
