Twenty-five to defend off 2 overs in an IPL game. Should your best bowler bowl the 19th over?

He could win you the game in the 19th over, or provide sufficient cushion for a weaker bowler to defend in the 20th.

The 20th over is going to be a high-pressure over, irrespective of what happens in the 19th, and the bowler most capable of handling that pressure should be held back for it.