Resemblance vs Similarity

Resemblance usually refers only to appearance. Similarity can be based upon appearance or any other attribute. Thus, your sentences are equivalent.

"… that they look almost identical?"

You look so much like John.

much : to a great extent; a great deal (used in comparison to other people who don't look like John.) "You look much like John (does)."
so : extremely; very much (used for emphasis): "She looked so pretty."

The 'so' emphasises 'much', to give the idea of 'nearly identical'.
You look so much like John!

A few points -

Proper nouns (including the names of people) start with a capital letter, so John not john

There are degrees of resemblance or similarity;

Slight resemblance, resemblance, strong or great resemblance; slight similarity, similarity, strong or great similarity. If something is "almost identical" to something else then the resemblance or similarity possibly needs emphasising with 'strong' or 'great', e.g. your face is very similar to John's, or your face very much resembles John's.

Finally, in ordinary conversation of the "Hello John" type, native speakers do not speak so formally. They would probably say something like "You look very much like John", as Bazza6 has noted.