• The Canon SL1 body costs $399.
• The Canon 5D Mk III body costs $2,599.

What is the difference in image quality and low light performance?

There is almost no difference in the test shown at the link.

Click through the link to see the photo comparisons for yourself.

Source: Canon 5D Mark III vs. SL1 Resolution and High ISO Comparison

There are two possible differences, not shown at the link. Larger sensors and lenses enable a narrower depth of field, all else being equal. Second, lens quality and sharpness matters and generally, less expensive lenses are less sharp.

For many use scenarios and many consumers, neither of those issues matters a lot.

Smaller and lighter cameras are much easier to carry. Size and weight – and your willingness to carry a lighter or heavier load – should be taken into account when making purchase decisions, together with your intended applications.

The point is that many consumers might be buying the wrong camera for their needs.