In the HTB documentation can be read:
Note: HTB as child of another HTB is NOT the same as class under another class within the same HTB. It is because when class in HTB can send it will send as soon as hardware equipment can. So that delay of underlimit class is limited only by equipment and not by ancestors. In HTB under HTB case the outer HTB simulates new hardware equipment with all consequences (larger delay)
EDIT: My question is: the delay for having a qdisc children of another is large enough for not considering this approach over having only one qdisc?
The thing is that I must have enabled QoS because I want prioritize some traffic over other one, I'm just asking for the approach to do that.
Thanks in advance.