The servo market is a war of brands, they try to lock you in a corner and buy into one brand only so that it becomes too expensive to switch to another brand. Now 3D printers are not RC toys, and when you only use one servo somewhere all of this seems a no-brainer. But the game changes once you start to make a kit, use multiple servos. And later on eventually scale up the dimensions of the kit, or want to make it move faster, etc. Once you change brand, then you also need to provide the parts that fit on that brand.

So all this in mind then you might say... Lets forget servos and stick with steppermotors. Specially with open hardware in mind this servo-slavery might just seem to be a thing to avoid.

Non the less, people use servos, and servos can offer a low cost, cheap and powerful solution.

There are many brands and  there are several small differences. The high-end brands like Futaba, Hitech, Ace, Traxxas often use better bearings, metal gears, and faster motors. There are Analog and Digital servos. And every brand has it's own specifications for the Hub couplers (servo horns),and they are not interchangeable. The number of teeth all vary.

To compare the gears bearings and and motors you can use http://www.servodatabase.com/

Though this database does not list the actual mechanical dimensions of the different types of hubs.