Hi. I have several sewing machines. They are quality machines and work very well. Since I cannot get what they are worth on a trade-in, I just keep them whenever I upgrade. I am a big fan of Pfaff and Janome machines and also a strong believer in buying from a reputable dealer if possible. He/she can help you find the machine that meets your needs.
For quilt-piecing and quilting, there are many important features to look for, several already mentioned in other responses to your question. One of my favorite features is the auto. needle up/down. Another important feature for me is that the needle position can be changed from center to left or right.
If you have the luxury of a local reputable dealer, previously owned machines are well worth checking out. They are traded in for various reasons and very often have nothing wrong with them. A good dealer will give used machines thorough maintenance before putting them up for sale. It's possible to get a fairly high-end used machine for about the same cost as a new basic machine and often comes with "extras" that the former owner has bought in addition to the standard accessories.
Best of luck! Weezie