This article is from the Photographic Lenses FAQ, by David Jacobson with numerous contributions by others.
Assuming the aperture diameter is the same in the two cases, the teleconverter case will require K^2 times the exposure while the enlarging case will enlarge the grain in the film K times as much. The teleconverter adds some aberrations of its own, while enlarging more will make aberrations in the enlarging lens more apparent. All other effects are identical. In 35mm format, grain is usually the dominate factor in image quality.