http://www.taunton.com/threads/pdf/S...chineChart.pdf has a comparison chart for sewing machines, but it all comes down to what you want and what you'll use.
Bring some scraps and batting to test machines you want to quilt with. If you are going to use it for a long time, you may want to go for more features than you currently use, just to stretch your abilities. Some accessories can really be pricey, so make sure to figure the cost in if not included.
Ask questions about the mechanics. I bought the commercial Singer, thinking it was all metal. First time it was in the shop, I found a lot of it was plastic painted to
look like metal, and the works were not nearly as heavy-duty as I had been led to believe.