Allow me to share what I know even though I have never taken
Qltt personally. From what I have heard,
Qltt is much much easier than New York Bar.
Passing New York Bar can help you find a job in London b/c it will allow you to work for international firms like Jones Day or consulting group like McKenzie
Also, passing the NY bar means that you can find a job in New York that requires a lot of traveling to UK (if that's what you are looking for)
If after studying
Qltt, you think it is demanding, then from what I have heard, New York Bar will be 5 times as demanding.
good luck!!