There are midarms (18" or smaller throat) and longarms (19" throat or larger). The bigger the throat, the more space you can quilt before having to roll the quilt.
There are many different features available such as stitch regulators, auto advances, channel locks, hydraulic lift tables, special wheels, special handles, special lights.
Tables differ. Weights differ. Speeds differ. Glide differs.
Choosing a machine is like choosing a car. It's not that one model will necessarily be BETTER than another; it will just be better for YOU. Only you can determine which machine is best suited to your needs.
My best advice is to do some research on the websites of the large companies: A-1 Quilting Machines, gammill, APQS, HandiQuilter. Then attend some shows where you can test drive the machines. And then talk with longarmers and ask why they bought what they bought. Most people love what they bought. They bought what they needed.
I know I love mine and it has been a workhorse for me. In 5 years of quilting a few hundred quilts a year, I've only had to replace the lightbulb and the checkspring.
Happy owner of an A-1 Elite 23" longarm