It depends on the movie! That's why it is impossible to be definitive.
I remember when I did 'Saving Pte Ryan'. A few moments into chapter two, when the waves are crashing onto the beach and foam was swirling around the anti-landing devices, I could pause the movie and just about count the block artefacts in the scene! (Anyone who has ever made their own movies and then used an MPEG encoder to prepare the footage for DVD video, will tell you that moving water is a real challenge. The restless surface of a swimming pool is next to impossible to render without artefacts).
Now 'Pte Ryan' is a long movie and all the transcoders I experimented with were really screwing the bitrate down and making the problem worse. The only one that did not exhibit this blockiness was InstantCopy.
But, scenes taken from other parts of the movie were indistinguishable whichever transcoder had been used!
This is why you must be sceptical when reading these sort of reports. By choosing appropriate material you can prove any transcoder to be better than another. Find your general favourite and use it all the time. If occasionally it stumbles over a tricky movie, then - and only then - try one of the others and see if it makes an improvement.