There are many ways to make chocolate from the bean. The route that most small startups take is using a so-called wet grinder as a "universal" - a grinder/refiner/conche. The reason for going this way is simple - it's very inexpensive to get started.
The wet grinder uses two grinding stones against a stone base. Pressure is exerted on the grinding stones which takes the very large particles of cocoa and sugar and refines them to the desired size while at the same time doing flavor and texture development.
Now, there's no need to fixate on 15 microns - it's not just the particle size, it's the particle size distribution, which looks a lot like a bell curve. No matter what you do in a wet mill you are going to get a distribution of particle sizes - some will be larger than the center/peak and some will be smaller. The location of the peak and the shape of the curve are related and important. It's also important not to have too many particles that are too small as this can make the chocolate gummy and pasty.
Now that that's out of the way.
The real engineering differences between the small tabletop Santhas and CocoaTowns are not all that great. They have small motors (too small), tend to have lousy drive belts, and plastic parts that wear down quickly. The main difference is in the way pressure is applied. The Santha has a knob you turn (adjustable) and the CocoaTown has a spring-loaded lid. There are lots of posts here on TheChocolateLife about the replacement belts and bushings for the small machines.
One of the main price differences you'll see between the CocoaTown and the Santhas has to do with size. The CocoaTown comes in one large size (a 65 liter working bowl). The Santha has more sizes; the 20 and 40 liter machines are the most common. The size difference accounts for most of the price difference.
Well - not really. When you buy a Santha Spectra 40 it comes with an electronic speed controller. When you buy a CocoaTown ECGC65L it does not.
So which is better? It's hard to say. If you are just starting out, you're probably best off with one or three of the small tabletop machines to learn the process so to speak. Once you're comfortable that you know what to do and that you have a business, it's really all about how much production you need. If you are doing 200kg a month that's one answer. If you're doing 200kg a week it's a different answer. If you're at 200kg/week or more you also want to start thinking about bringing other machinery into the mix - a pre-grinder for the nibs and sugar and a dedicated conche so you're using the wet mills only as grinder/refiners -- or, you've moved on to an entirely new suite of equipment.