A self-lubricating machine saves time and eliminates the risk of staining the fabric with oil. Automatic tie-off secures threads at the end of stiching without backstiching. Automatic needle stop saves you from turning the handwheel to raise the needle out of the fabric to remove work from the machine. Many machines also offer the option of automatic stop down to allow you to keep both hands on your work when pivoting frequently, as for topstiching and aplique. A power swithch that automatically turns the machine on and off is an important safety consideration.
Some machines only have a light switch, in which case the mahcine is off only when it is unplugged.
Each machine has its own touch and feel. Gather fabric samples of various weights and fibers. Look for uniform and well balanced stitches. The bobbin and needle threads should interlock between the fabric layers, rather than looping on either side, and the fabric should lie pucker-free. Not only should the stitches look good, but the quality should remain consistent when you lengthen the stitches or change sewing speed.