Tzhaar ket-om is better then dragon longsword well because dragon longsword gives a 80+ strenght bonus but tzhaar give 85+.
Also, dragon longsword lowers some stats but tzhaar doesnt.
Keep in mind that the dragon longsword is faster and has a special attack. The tzhaar-ket-om's slower speed gives it a disadvantage. If you were to have a test where people with identical stats were fighting with only one item each (dragon longsword and tzhaar-ket-om), without armor, the person with the dragon longsword would most likely win.