Both are interchangeable. The second example ends with the preposition "of". This is what's called a "stranded preposition", and it used to be thought of as being completely wrong. These days, however, most people view it as being perfectly acceptable.
So far as which sentence is more "natural", I would say that neither is particularly natural. If I were writing this - and, particularly if I was speaking it - I would split it into two sentences, as "He mentioned the names of a few people. I didn't think highly of any of them".
Student or Learner