D & I are practically the same in everyway apart from price. If you call CX direct in Taipei they will book it for you where a TA might not have access to it, also TA in Taiwan are notorious for saying the fare is not there so they can get more commission and sell you more expensive tickets. There is a company who I found who advertise on FT called Whose travel in Taichung and they put me onto this and saved me a fortune.