A monogramming machine is a specialized version of an embroidering machine. Commercial embroidery devices are multipurpose machines that can do monogramming plus many other types of embroidery. Monogramming machines are limited to single functions of putting monogrammed letters or symbols on apparel or fabric. How can you decide which type to purchase? First you have to look at the volume, complexity of the items you need monogrammed, and the quality level of the monogrammed output. Once you have evaluated those you can focus in on the type of machine to buy. Which one you buy is driven by the ability to do the functions that are important to you. If you are only interested in monogramming you will be able to find many machines on eBay or Amazon that will suit your fancy.
If however you need to do more than the monogramming function, you will need to focus on commercial embroidery machines. You will wind up here if you have high volumes, complex designs, and need the ability to work with multiple threads and digitized images. Some commercial embroidery machines are known as hybrid machines because they can handle regular sewing as well as embroidery. These are the junior versions of the commercial embroidery ones. The full-fledged embroidery machines are industrial strength. Make certain that you evaluate the durability of the machine. Monogramming machines can be regular sewing machines with monogramming ability, hybrid machines that can do both monogramming and some embroidery, or full-fledged commercial embroidery machines capable of embroidering continuously. Don't underbuy or overbuy for your needs. Evaluate carefully. The quality of the output, the durability of the machine, the functions it must perform, and, of course, price are all important factors in your decision. Another one should be the ability to get the machine serviced or repaired periodically. It doesn't matter how good it is if it's broken or not working smoothly.