Should I use what the professor says to me in the email as a material to write in the SOP?
Yes, you should delete that. It doesn't add value. You should mention that you were accepted and what you plan to do, but there is no need to include the following smalltalk. Getting the Prof to say "You would be a terrific addition to our group!" is politeness not an achievement in itself. An achievement would be if he said that after you worked there for a month.

Also, I don't think that is the only thing that makes you stand out. The Prof surely has reasons why he accepted you, he doesn't accept at random. Maybe you could ask him what led to that decision.

How is the following:

Professor X wholeheartedly accepted me to work on the relation between 
A and B, which will help to formulate C theory. I very much look forward
to work on this topic because ...