I can say that, having met both men multpile times, George Takei is by far the more gracious and genuine person. My wife and I spent a decent amount of time with him and Brad at the convention, and they were just really kind, funny, interesting people.
The anecdotes seem to universally agree that Takei has always been fantastic at in-person congeniality with the fans, and Shatner largely hasn't been.
On screen though, the supporting cast got more wooden with each movie, and to top it off, they were poorly served by dialog that was by turns stiff, silly, or transparently unnecessary in the plot. Between their own declining acting craft and the bad writing they were given, it was hopeless.