Connie Keller is my APQS dealer and sold a Lenni to me. This is the smallest and least expensive of the APQS machines. A certification class doesn't take much time -- or require much cash investment -- and I think you are right to take such a class. Such a class would give you the opportunity to use several longarm machine models, to talk to people who quilt for others, and learn about the options for each type of machine that you might want to consider in making the purchase. Connie Keller has all APQS models in her store. She can also demonstrate the APQS software that drives that brand of machine in case you are interested that upgrade. (I have it, I love it!)
Connie is a great gal, will take all the time you need to get comfortable with the machines, and has a wonderful, bubbly personality to boot.
I don't know Nanc Christopherson, so I cannot comment on her certification courses.