Has this changed? (Have the delegate libraries gotten better?)http://blog.endpoint.com/2009/12/jpeg-compression-quality-or-quantity.html
suggests that the difference is not that big. On the other hand, some other recent posts suggest that IM is still somewhat inferior w.r.t. JPEG.
Would it be possible to improve matters by, say, combining quantization with lossless or lossy JPEG compression (for thumbnails, esp.)? (I imagine this is hugely complicated.)