I think Shatner was a little more accomplished as an actor, but Williams would have been very good in his own way. Both men made a conscious effort to move with a cool, catlike, athletic gracefulness. Maybe Williams would have played Kirk overall in way similar to Shatner. Leonard Nimoy might have found the same kind of "space" in which to fulfill Spock's potential.
Yeah I haven't watched LIS or Zorro in awhile so I don't remember what kind of acting range he had, but I can definitely see him having much the same kind of energy and commanding presence as Shatner. And the dynamic between him and Nimoy probably would have been pretty similar as well.
At the very least, it would have been cool to see him as a fellow Starship captain in an episode.