user13147: 
Between 18. Na2 and 18. Rac1, which is more favorable for white?

user-1: IM Silman gives this position as problem 3 in The reassess your chess workbook. He recommends 18. Na2:

Short played the surprising and strong 18. Na2!!. This idea ... shows that White considers the b4-Knight to be more important than its brother on e4. The reason is simple: the b4-Knight is the main guardian of Black's weak points on a6, c6 and d5.

IM Silman does not consider 18. Rac1 in his analysis.

user12540: I think Na2 is the best though it takes the Knight away from the Centre of the board hindering basic Chess Principles but if you take a note then Black is ok on Centre and K-side and his weakness resides in the Queenside . 
So Na2 removes the Knight from b4 and the Bishop can dominate on light Squares like c6 where it can attack the d5 Square . Also the a6 Pawn will become weak once the Knight is lost .

13147: <p><a href="https://i.stack.imgur.com/dqCVh.png" rel="noreferrer"><img src="https://i.stack.imgur.com/dqCVh.png" alt="enter image description here"></a></p>

<p>Between 18. Na2 and 18. Rac1, which is more favorable for white?</p>
 -1: <p>IM Silman gives this position as problem 3 in <em>The reassess your chess workbook</em>. He recommends 18. Na2:</p>

<blockquote>
  <p>Short played the surprising and strong <strong>18. Na2!!</strong>. This idea ... shows that White considers the b4-Knight to be more important than its brother on e4. The reason is simple: the b4-Knight is the main guardian of Black's weak points on a6, c6 and d5.</p>
</blockquote>

<p>IM Silman does not consider 18. Rac1 in his analysis.</p>
 12540: <p>I think Na2 is the best though it takes the Knight away from the Centre of the board hindering basic Chess Principles but if you take a note then Black is ok on Centre and K-side and his weakness resides in the Queenside . </p>

<p>So Na2 removes the Knight from b4 and the Bishop can dominate on light Squares like c6 where it can attack the d5 Square . Also the a6 Pawn will become weak once the Knight is lost .</p>
