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