Quality first, then communication, cost and turnaround time. I'd want to see previous work or photos to determine quality. It would be best if the long arm machine has a stitch regulator since that improves the quality of the stitches.
It's very important that the person you speak to is the one doing the quilting. Some places hire people to run their machines but they aren't the ones you speak to. I also don't like pantographs or machine designs. I want my quilts to look like a person quilted it. Guess that's one reason that I finally invested in my machine.