Emma considers Mr Knightley more amiable than his brother as she considers this in chapter 34 She thought it in reality a sad exchange for herself, to have him with his grave looks and reluctant conversation opposed to her instead of his brother.

I thought Mr Knightley also seems to keep well away from Jane Fairfax this time - perhaps trying to reduce further gossip concerning himself and Jane Fairfax.

Mr Knightley's mood seems to change during the dinner party - here is a collection of quotes concerning him chp 34-36
"Yes," said his brother hesitatingly, "there is a likeness. I know what you mean -- but Emma's hand is the strongest." chp34

Mrs. Weston was disengaged and Emma began again -- "Mr. Frank Churchill writes one of the best gentlemen's hands I ever saw."

"I do not admire it," said Mr. Knightley. "It is too small -- wants strength. It is like a woman's writing."
and
"Well, well, I have that note; and can shew it after dinner to convince Mr. Knightley."

"Oh! when a gallant young man, like Mr. Frank Churchill," said Mr. Knightley drily, "writes to a fair lady like Miss Woodhouse, he will, of course, put forth his best."

On the news that Frank Churchill will be returning to Highbury: It was well that he took every body's joy for granted, or he might not have thought either Mr. Woodhouse or Mr. Knightley particularly delighted. They were the first entitled, after Mrs. Weston and Emma, to be made happy. From them he would have proceeded to Miss Fairfax, but she was so deep in conversation with John Knightley, that it would have been too positive an interruption; and finding himself close to Mrs. Elton, and her attention disengaged, he necessarily began on the subject with her. Chp 35

The remaining five were left to their own powers, and Emma doubted their getting on very well; for Mr. Knightley seemed little disposed for conversation;...
and
Mr. John Knightley proved more talkative than his brother. chp 36

It seems Mr Knightley has a very deep dislike of Frank - I wonder why he would be at such pains to dissaprove of him. I think Frank Churchill poses some threat to Mr Knightley, otherwise he would simply have noted Frank's weakness, but he would not ruminating on them as would something that matters.

Mr Knightley seems to imply here that he does not plan to be so often part of the gaieties that is probably going to include Frank Churchill.
Mr J Knightley: "...A little while ago, every letter to Isabella brought an account of fresh gaieties; dinners at Mr. Cole's, or balls at the Crown. The difference which Randalls, Randalls alone makes in your goings-on, is very great."

"Yes," said his brother quickly, "it is Randalls that does it all."

"Very well; and as Randalls, I suppose, is not likely to have less influence than heretofore, it strikes me as a possible thing, Emma, that Henry and John may be sometimes in the way. And if they are, I only beg you to send them home." chp36

Is Mr John Knightley hinting here at the possible match between Frank and Emma?

"No," cried Mr. Knightley, "that need not be the consequence. Let them be sent to Donwell. I shall certainly be at leisure."

But Mr Knightley seems to cheer up after this exchange:

"Upon my word," exclaimed Emma, "you amuse me! I should like to know how many of all my numerous engagements take place without your being of the party; and why I am to be supposed in danger of wanting leisure to attend to the little boys. These amazing engagements of mine -- what have they been? Dining once with the Coles -- and having a ball talked of, which never took place. I can understand you -- (nodding at Mr. John Knightley) -- your good fortune in meeting with so many of your friends at once here, delights you too much to pass unnoticed. But you, (turning to Mr. Knightley,) who know how very, very seldom I am ever two hours from Hartfield, why should you foresee such a series of dissipation for me, I cannot imagine. And as to my dear little boys, I must say, that if aunt Emma has not time for them, I do not think they would fare much better with uncle Knightley, who is absent from home about five hours where she is absent one -- and who, when he is at home, is either reading to himself or settling his accounts."

Mr. Knightley seemed to be trying not to smile; and succeeded without difficulty, upon Mrs. Elton's beginning to talk to him. chp36

I can't help but smile too when Emma manages to throw Mr Knightley's previous comment that he'd rather go over accounts than dance back at him. I think Emma and Mr Knightley are a good intellectual match. IMHO there are no exchanges so clever as between Emma and Mr Knightley and nobody but Mr Knightley really beats Emma at her own game - although I must say Frank is quite good at figuring out what goes on in Emma's head, but he uses it (like their speculations on Jane Fairfax)to lead her further into her fancyful thinking instead of talking her sensibly out of it.