so then is it better to transform ardr tokens into nxt?
Why?
Why is ARDR more valuable that NXT on coinmarketcap?
Because that's what the market thinks it should be valued at.
Ardor will be the parent chain. One could see it as more important. But we'll have to see if that turns out true for the value. Ardor is quite limited, and only really there for forging the chain.
Child-chains like IGNIS have many more features.
NXT will keep existing and might have it's specific uses. And you get 0.5 IGNIS for it at the air-drop. So it might be a great deal. But yeah, at the end it's up to you to predict what's really the best choice here.