Iroh is more versatile and skilled and the better strategic thinker. He's also good at conserving his energy and before the liberation of Ba Sing Se we never see him use his full power, ever. He always uses the minimum power necessary to accomplish what he wanted.

Why is Iroh wise?

Much like his belief that failure allowed someone to begin again, Iroh knew that he could learn from his mistakes and the tragedy in his life. He bad experiences became learning opportunities, striving to become better. He wanted the same for his nephew, which is likely why so much of his wisdom was aimed at Zuko.

Why is Iroh so powerful?

5 Most Powerful: Iroh

Iroh trained with the last dragons, and he displayed firebending prowess and technique that earned him international acclaim and fear. He hardly fought after the loss of Lu Ten, but on the few occasions he showed aggression, he proved he could match with the best.