The difference? The iPhone-version (or 'high end', because it works on my Android-devices as well) looked much more beautiful, but m-one has all the features one would expect from a mobile version of the site up. Take a look at this screenshot (low quality);
In order for the house on M and the iPhone:
facebook faceoff: house
Facebook profile in the M and the iPhone:
facebook faceoff: Profile
Up friends in the M and the iPhone:
facebook faceoff: Friends
Basically, except for the default page-friend, iphone.facebook.com not in terms of offering more or better functionality than the version of the "low end". In contrast, the iPhone-version does not even come close in comparison with m.facebook.com.
Dear Facebook; bling is not everything and I really do not want to install your application, so kudos to create a site, feature-rich mobile strong in m.facebook.com, but most do not implement the same functionality on the iphone. facebook.com.