Check the reviews on these models at
http://www.patternreview.com .
I would be lost now without the needle-down feature, but it's true that all you need is a machine that does a decent straight stitch.
The other most important feature, IMO, is how well the machine keeps the tension. You can test this by bringing your own fabrics; ideally the machine should not need any adjusting (other than stitch length) when you change from one layer of cotton to 4 layers of denim. If you can vary the layers and types of fabrics back and forth without the machine being "touchy", chances are it will hold its tension well.
The other thing I would do is bring an actual test quilt sandwich with you, using the batting you like the best. See how the stitches look both top and underneath when you quilt with the machine. Use different colors of thread on top and in the bobbin to get a clear view of how well-balanced the stitches are when working on a quilt sandwich. Free-motion quilt on the sandwich too. Some machines handle FMQ better than others.
The Janome Jem is a great machine for portability if you intend to take quilting classes. However, a 3/4-size machine will have less area under the arm for handling large quilts. If you are young enough to haul a sewing machine around on a luggage dolly, you might want to stick with a full-sized machine.