I think McGillion might've done better, personally, but I'm eager to see how Pegg handles the role.
I'm not sure what you mean by "might have done better". It is not his call as to what role he gets to play. He can only say "Yes" or "No" to whatever J.J. Abrams offers him. Personally, I think he would have made the perfect Scotty. And I'm looking forward to seeing more of him on Stargate Atlantis.