In similar figures, angles or sides are proportional, but in congruence, figures are exactly the same- angles, sides, and everything.
Additionally, similar figures always have the same shape, but not always the same size. But in congruence, figures always have the same shape AND size.
So, corresponding sides for similar figures should be proportional (that is, have the same scale factor. And corresponding sides in congruent figures must be exactly the same.
Hope that helps!